Marine Rhizopods
When we stand on a beach of fine sand on a very calm day watching the progress of the ripples over the sand as the tide recedes we frequently observe whitish lines marking the limits reached by the successive ripples as they advance toward the shore. If, now, we scrape up a little of the surface sand, following the exact course of one of these whitish streaks, and examine the material obtained by the aid of a good lens, we shall in all probability discover a number of minute shells among the grains of sand.
These shells are of various shapes—little spheres, discs, rods, spirals, &c.; but all resemble each other in that they are perforated with a number of minute holes or foramina. They are the skeletons of protozoons, belonging to the class Rhizopoda, and they exist in enormous quantities on the beds of certain seas.
Fig. 54.—A group of Foraminifers, magnified
We will first examine the shells, and then study the nature of the little animals that inhabit them.
The shells vary very much in general appearance as well as in shape. Some are of an opaque, dead white, the surface somewhat resembling that of a piece of unglazed porcelain; others more nearly resemble glazed porcelain, while some present quite a vitreous appearance, much after the nature of opal. In all cases, however, the material is the same, all the shells consisting of carbonate of lime, having thus the same chemical composition as chalk, limestones, and marble.
If hydrochloric acid be added to some of these shells, they are immediately attacked by the acid and are dissolved in a very short time, the solution being accompanied by an effervescence due to the escape of carbonic acid gas.
The shells vary in size from about one-twelfth to one three-hundredth of an inch, and consist either of a single chamber, or of many chambers separated from each other by perforated partitions of the same material. Sometimes these chambers are arranged in a straight line, but more frequently in the form of a single or double spiral. In some cases, however, the arrangement of chambers is very complex.
We have already referred to the fact that the shells present a number of perforations on the exterior, in addition to those which pierce the partitions within, and it is this characteristic which has led to the application of the name Foraminifera (hole-bearing) to the little beings we are considering.
Fig. 55.—A Spiral Foraminifer Shell
Fig. 56.—A Foraminifer out of its shell
The animal inhabiting the shell is exceedingly simple in structure, even more so than the amœba. It is merely a speck of protoplasm, exhibiting hardly any differentiation—nothing, in fact, save a contractile cavity (the vacuole), and numerous granules that probably represent the indigestible fragments of its food.
The protoplasm fills the shell, and also forms a complete gelatinous covering on the outside, when the animal is alive; and the vacuole and granules circulate somewhat freely within the semi-solid mass. Further, the protoplasm itself is highly contractile, as may be proved by witnessing the rapidity with which the animal can change its form.
When the foraminifer is alive, it floats freely in the sea, with a comparatively long and slender thread of its substance protruded through each hole in the shell. These threads correspond exactly in function with the blunt pseudopodia of the amœba. Should they come in contact with a particle of suitable food-material, they immediately surround it, and rapidly retracting, draw the particle to the surface of the body. The threads then completely envelop the food, coalescing as soon as they touch, thus bringing it within the animal.
Fig. 57.—The same Foraminifer (Fig. 56) as seen when alive
Fig. 58.—Section of the Shell of a Compound Foraminifer
The foraminifer multiplies by fission, or by a process of budding. In some species the division of the protoplasm is complete, as in the case of amœbæ, so that each animal has its own shell which encloses a single chamber, but in most cases the ‘bud’ remains attached to a parent cell, and develops a shell that is also fixed to the shell of its progenitor. The younger animal thus produced from the bud gives rise to another, which develops in the same manner; and this process continues, the new bud being always produced on the newest end, till, at last, a kind of colony of protozoons is formed, their shells remaining attached to one another, thus producing a compound shell, composed of several chambers, arranged in the form of a line or spiral, and communicating by means of their perforated partitions. It will now be seen that each ‘cell’ of the compound protozoon feeds not only for itself, but for all the members of its colony, since the nourishment imbibed by any one is capable of diffusion into the surrounding chambers, the protoplasm of the whole forming one continuous mass by means of the perforated partitions of the complex skeleton.
Fig. 59.—Section of a Nummulite Shell
Some of the simplest foraminifers possess only one hole in the shell, and, consequently, are enabled to throw off pseudopods from one side of the body only. In others, of a much more complex nature, the new chambers form a spiral in such a manner that they overlap and entirely conceal those previously built; and the development may proceed until a comparatively large discoid shell is the result. This is the case with Nummulites, so called on account of the fancied resemblance to coins. Further, some species of foraminifera produce a skeleton that is horny in character, instead of being calcareous, while others are protected merely by grains of sand or particles of other solid matter that adhere to the surface of their glutinous bodies.
Fig. 60.—Globigerina bulloides, as seen when alive, magnified
We have spoken of foraminifera as floating freely about in the sea water, but while it is certain that many of them live at or near the surface, some are known to thrive at considerable depths; and those who desire to study the various forms of these interesting creatures should search among dredgings whenever an opportunity occurs. Living specimens, whenever obtained, should be examined in sea water, in order that the motions of their pseudopods may be seen.
If we brush off fragments from the surface of a freshly broken piece of chalk, and allow them to fall into a vessel of water, and then examine the sediment under the microscope, we shall observe that this sediment consists of minute shells, and fragments of shells, of foraminifers. In fact, our chalk beds, as well as the beds of certain limestones, consist mainly of vast deposits of the shells of extinct foraminifera that at one time covered the floor of the sea. Such deposits are still being formed, notably that which now covers a vast area of the bed of the Atlantic Ocean at a depth varying from about 300 to 3,000 fathoms. This deposit consists mainly of the shells of a foraminifer called Globigerina bulloides, a figure of which is given on the opposite page.
Fig. 61.—Section of a piece of Nummulitic Limestone
The structure of chalk may be beautifully revealed by soaking a small piece of the rock for some time in a solution of Canada balsam, allowing it to become thoroughly dry, and then grinding it down till a very thin section is obtained. Such a section, when viewed under the low power of a compound microscope, will be seen to consist very largely of minute shells; though, of course, the shells themselves will be seen in section only.
The extensive beds of nummulitic limestones found in various parts of South Europe and North Africa are also composed largely of foraminifer shells, the most conspicuous of which are those already referred to as nummulites—disc-shaped shells of a spiral form, in which the older chambers overlap and hide those that enclose the earlier portion of the colony.
Before concluding our brief account of these interesting marine protozoons, it may be well to point out that, although the foraminifera belong to the lowest class of the lowest sub-kingdom of animals, yet there are some rhizopods—the Monera, which are even simpler in structure. These are mere specks of undifferentiated protoplasm, not protected by any shell, and not even possessing a nucleus, and are the simplest of all animal beings.
The second division of the Protozoa—the class Protoplasta—has already received a small share of attention, inasmuch as the amœba, which was briefly described as a type of the whole sub-kingdom, belongs to it.
The study of the amœba is usually pursued by means of specimens obtained from fresh-water pools, and reference has been made to it in a former work dealing particularly with the life of ponds and streams; but it should be observed that the amœba inhabits salt water also, and will be frequently met with by those who search for the microscopic life of the sea, especially when the water examined has been taken from those sheltered nooks of a rocky coast that are protected from the direct action of the waves, or from the little pools that are so far from the reach of the tides as to be only occasionally disturbed. Here the amœba may be seen creeping slowly over the slender green threads of the confervæ that surround the margin of the pool.
The third class—Radiolaria—is of great interest to the student of marine life, on account of the great beauty of the shells; but, as with the other members of this sub-kingdom, a compound microscope is necessary for the study of them.
The animals of this group resemble the foraminifers in that they throw out fine thread-like pseudopods, but they are distinguished from them by the possession of a membranous capsule in the centre of the body, surrounding the nucleus, and perforated in order to preserve the continuity of the deeper with the surrounding protoplasm. They have often a central contractile cavity, and further show their claim to a higher position in the animal scale than the preceding classes by the possession of little masses of cells and a certain amount of fatty and colouring matter.
Fig. 62.—A Group of Radiolarian Shells, magnified
Some of the radiolarians live at or near the surface of the ocean, while others thrive only at the bottom. The former, in some cases, appear to avoid the light, rising to the surface after sunset; and it is supposed that the phosphorescence of the sea is due in part to the presence of these animals. The latter may be obtained from all depths, down to several thousand fathoms.
The beauty of the radiolarians as a class lies in the wonderful shells that protect the great majority of them. These shells are composed not of carbonate of lime, as is the case with foraminifers, but of silex or silica, a substance that is not acted on by the strongest mineral acids. They are of the most exquisite shapes, and exhibit a great variety of forms. Some resemble beautifully sculptured spheres, boxes, bells, cups, &c.; while others may be likened to baskets of various ornamental design. In every case the siliceous framework consists of a number of clusters of radiating rods, all united by slender intertwining threads.
It is not all the radiolarians, however, that produce these beautiful siliceous shells. A few have no skeleton of any kind, while others are supported by a framework composed of a horny material, but yet transparent and glassy in appearance.
The sizes of the shells vary from about one five-hundredth to one half of an inch; but, of course, the larger shells are those of colonies of radiolarians, and not of single individuals, just as we observed was the case with the foraminifers.
Those in search of radiolaria for examination and study should, whenever possible, obtain small quantities of the dredgings from deep water. Material brought up by the trawl will often afford specimens; but, failing these sources of supply, the muddy deposit from deep niches between the rocks at low-water mark will often provide a very interesting variety.
Place the mud in a glass vessel, and pour on it some nitric acid (aqua-fortis). This will soon dissolve all calcareous matter present, and also destroy any organic material. A process of very careful washing is now necessary. Fill up the vessel with water, and allow some time for sedimentary matter to settle. Now decant off the greater part of the water, and repeat the process several times. By this means we get rid of the greater part of the organic material, as well as of the mineral matter that has been attacked by the acid; and if we examine the final sediment under the microscope, preferably in a drop of water, and covered with a cover-glass, any radiolarians present will soon reveal themselves.
It is often possible to obtain radiolarian shells, as well as other siliceous skeletons, through the agency of certain marine animals. The bivalve molluscs, for example, feed almost entirely on microscopic organisms; and, by removing such animals from their shells, and then destroying their bodies with aqua-fortis, we may frequently obtain a sediment composed partly of the skeletons referred to.
There remains one other class of protozoons to be considered, viz. the Infusorians—the highest class of the sub-kingdom. In this group we observe a distinct advance in organisation; for, in the first place, the infusorians are enclosed in a firm cuticle or skin, which forms an almost complete protective layer. Within this is a layer of moderately firm protoplasm, containing one or more cavities that contract at intervals like a heart. Then, in the interior, there is a mass of softer material with cavities filled with fluid, two solid bodies, and numerous granules.
Fig. 63.—Three Infusorians magnified
In these creatures we find, too, a distinct and permanent mouth, usually funnel-shaped, leading to the soft, interior substance, in which the food material becomes embedded while the process of digestion proceeds. Here, then, for the first time, we meet with a special portion of the body set apart for the performance of the work of a stomach; and, further, the process of digestion being over, the indigestible matter is ejected through a second permanent opening in the exterior cuticle.
Again, the infusorian does not move by means of temporary pseudopods, as is the case with the lower protozoons, but by means of minute hair-like processes which permanently cover either the whole of the body, or are restricted to certain portions only. These little processes, which are called cilia, move to and fro with such rapidity that they are hardly visible; and, by means of them the little infusorian is enabled to move about in its watery home with considerable speed.
In some species a few of the cilia are much larger than the others, and formed of a firmer material. These often serve the purpose of feet, and are also used as a means by which the little animal can anchor itself to solid substances.
As with the lower protozoons, the infusoria multiply by division; but, in addition to this, the nucleus may sometimes be seen to divide up into a number of minute egg-like bodies, each of which, when set free, is capable of developing into a new animal. Should the water in which infusorians have been living evaporate to dryness, the little bodies just mentioned become so many dust particles that may be carried away by air currents; but, although dry, they retain their vitality, and develop almost immediately on being carried into a suitable environment.
Infusorians are so called because they develop rapidly in infusions of various vegetable substances; and those who desire to study their structure and movements with the aid of a microscope cannot do much better than make an infusion by pouring boiling water on fragments of dried grass, and leaving it exposed for a few days to the warm summer atmosphere. The numerous germs floating in the air will soon give rise to abundance of life, including several different species of infusoria, varying from 1/30 to 1/2000 of an inch in length.
Fig. 64.—A Phosphorescent Marine Infusorian (Noctiluca), magnified
Fresh-water pools and marshes provide such an abundance of infusoria that the animals are generally obtained for study from these sources, and a few of the common and most interesting species inhabiting fresh water have already been described in a former work. Nevertheless, the sea is abundantly supplied with representatives of the class, and it is certain that the beautiful phosphorescence sometimes observed in the sea at night is in part due to the presence of luminous infusoria, some of which appear to have an aversion to sunlight, retiring to a depth during the day, but rising to the surface again after sunset.
CHAPTER VIII
BRITISH SPONGES
It seems to be the popular opinion that sponges are essentially natives of the warmer seas, and it will probably be a surprise to many young amateur naturalists to learn that there are about three hundred species of this sub-kingdom of the animal world to be found on our own shores. It must not be thought, however, that they are all comparable with the well-known toilet sponges in regard to either size or general form and structure, for some of them are very small objects, no larger than about one-twentieth of an inch in diameter, and some form mere incrustations of various dimensions on the surfaces of rocks and weeds, often of such general appearance that they would hardly be regarded as animal structures by those who have not studied the peculiarities of the group.
Sponges are known collectively as the Porifera or Polystomata, and constitute a separate sub-kingdom of animals of such distinct features that they are not readily confused with the creatures of any other group. Their principal characteristic is expressed by both the group names just given, the former of which signifies ‘hole-bearing,’ and the latter ‘many openings’; for in all the members of the sub-kingdom there are a number of holes or pores providing a means of communication between the body cavity or cavities and the surrounding water. Most of these holes are very small, but there is always at least one opening of a larger size at the anterior end.
It will be seen from what we have just stated that sponges exhibit a distinctly higher organisation than the protozoa described in the last chapter, inasmuch as they possess a permanent body-cavity that communicates with the exterior; but in addition to this there are many points of differentiation of structure that denote a superior position in the scale of life.
In order to ascertain the general features of a sponge we cannot do better than select one of the simplest forms from our own shores. If we place the live animal in a glass vessel of sea water, and examine it with a suitable magnifying power, we observe a number of minute pores scattered over its whole surface; and a much larger opening at the free end. The animal is motionless, and exhibits no signs of life except that it may contract slightly when touched. The water surrounding the sponge also appears to be perfectly still, but if we introduce some fine insoluble powder, such as precipitated chalk, or a drop of a soluble dye, the motion of the suspended or soluble material will show that the water is passing into the sponge through all the small pores, and that it is ejected through the larger opening.
Fig. 65.—Section of a Simple Sponge
On touching the sponge we observe that it is of a soft, gelatinous consistence throughout, or if, as is often the case, the body is supported by a skeleton of greater or less firmness, a gentle application of the finger will still show that this framework is surrounded by material of a jelly-like nature. This gelatinous substance is the animal itself, and a microscopic examination will show that its body-wall is made up of two distinct layers, the inner consisting of cells, many of which possess a cilium or whip-like filament that protrudes from a kind of collar, its free extremity extending into the body-cavity.
These minute cilia are the means by which the water currents just described are set up. By a constant lashing movement they urge the fluid contained in the body-cavity towards the larger hole, thus causing the water to flow in through the numerous small pores. This circulation of sea water through the body-cavity of the sponge is the means by which the animal is supplied with air and food. Air is, of course, absorbed from the water by the soft material of the external layer of the body, but the constant flow of fresh water through the body-cavity enables this process of respiration to go on with equal freedom in the interior. The mode of feeding of the sponge is very similar to that of the protozoa. Organic particles that are carried into the body-cavity, on coming in contact with the cells of the internal layer, are absorbed into their protoplasm by which they are digested. Thus the sponge may be compared to a mass of protozoon cells, all united into a common colony by a more or less perfect coalescing of the cell-substance, some of the units being modified in structure for the performance of definite functions. The air and food absorbed by any one cell may pass readily into the surrounding cells, and thus each one may be said to work for the common weal.
Fig. 66.—Diagrammatic section of a portion of a Complex Sponge
The description just given applies only to the simplest of the sponges, and we have now to learn that in the higher members of the group the structure is much more complicated. In these the surface-pores are the extremities of very narrow tubes which perforate both layers of the body-wall and then communicate with wider tubes or spaces within, some of which are lined with the ciliated cells above described. These spaces, which are sometimes nearly globular in form, and often arranged in groups with a common cavity, communicate with wider tubes which join together until, finally, they terminate in a large opening seen on the exterior of the sponge. Hence it will be seen that the water entering the minute pores of the surface has to circulate through a complicated system of channels and spaces, some of which are lined with the ciliated cells that urge the current onwards before it is expelled through the large hole. Further, imagine a number of such structures as we have described growing side by side, their masses coalescing into one whole, their inner tubes and spaces united into one complex system by numerous inter-communications, and having several large holes for the exit of the circulating water, and you then have some idea of the general nature of many of the more complex sponges to be found on our shores (see fig. 66).
Fig. 67.—Horny Network of a Sponge, magnified
But even this is not all, for as yet we have been regarding the sponges as consisting of animal matter only, whereas nearly all of them possess some kind of internal skeleton for the support of the soft, gelatinous animal substance. The skeleton consists of matter secreted by certain cells from material in the water and food, and is either horny, calcareous, or siliceous. The horny skeleton is formed of a network of fibres of a somewhat silky character, and often, as in the case of the toilet sponges, highly elastic; but it is sometimes so brittle that the sponge mass is easily broken when bent. The fibres of this framework support not only the outer wall of the sponge, but also the walls of all the internal tubes and spaces, which are often of so soft a nature that they would collapse without its aid.
The other forms of skeletons consist of minute bodies of carbonate of lime or of silica, respectively, which assume certain definite shapes, resembling stars, anchors, hooks, pins, spindles, &c., and are known as spicules. Such spicules are usually present in those sponges that have horny skeletons, but in others they form the entire skeleton.
Sponges sometimes increase by division, a part being separated from the parent mass and then developing into a complete colony; and they may be reproduced artificially to almost any extent by this method, each piece cut off, however small, producing a new sponge. They also increase by a process of ‘budding,’ the buds produced sometimes remaining attached to the original colony, thus increasing its size, but on other occasions becoming detached for the formation of new colonies on a different site. In addition to these methods of reproduction there are special cells in a sponge that possess the function of producing eggs which are ejected through the larger holes. The eggs are usually developed in the autumn, and, after being ejected, swim about freely for a time, after which they become fixed to rocks or weeds, and produce sponges in the following year. The eggs may often be seen towards the end of the summer by cutting through a sponge, or by carefully pulling it asunder. They are little rounded or oval bodies, of a yellowish or brownish colour, distinctly visible to the naked eye, occupying cavities in the interior.
Sponges are classified according to the composition of the skeleton and the forms of the spicules, the chief divisions being:—
1. The Calcareous Sponges (Calcarea). Skeleton consisting of spicules of carbonate of lime in the form of needles and three-or four-rayed stars.
2. The Six-Rayed Sponges (Hexactinellida). Skeleton of six-rayed glassy spicules.
3. Common Sponges (Demospongia). Skeleton horny, flinty, or entirely absent.
The first of these divisions contains about a dozen known British species, which are to be found on the rockiest shores, attached to stones, weeds, or shells, generally hidden in very secluded holes or crevices, or sheltered from the light by the pendulous weeds. They should be searched for at the lowest spring tide, particular attention being given to the under surfaces of large stones, narrow, dark crevices, and the roofs of small, sheltered caves. They may be readily recognised as sponges by the numerous pores on the surface, though these are often hardly visible without a lens, and the calcareous nature of the skeleton may be proved by dropping a specimen into dilute hydrochloric acid, when the carbonate of lime will speedily dissolve, the action being accompanied by the evolution of bubbles of carbonic acid gas.
If calcareous sponges are to be preserved for future reference, they may be placed in diluted spirit, in which case the animal matter, as well as the mineral substance, will be preserved with but little alteration in the natural appearance and structure. A specimen which has been decalcified by means of acid, as above described, may also be preserved in the same manner; and small portions of this will serve for the microscopic study of the animal portion of the sponge. If the skeleton only is required, the sponge is simply allowed to dry, when the soft animal substance, on losing its contained water, will leave hardly any residue; or, better, allow the calcareous sponge to macerate in water for some days for the animal substance to decompose, and then, after a few minutes in running water, set it aside to dry.
Fig. 68.—Grantia compressa
Fig. 69.—Spicules of Grantia, magnified
Small portions of the skeleton, examined under the microscope, will show the nature of the calcareous spicules of which it is composed. These consist of minute needles and stars, the latter having generally either three or four rays.
We give figures of three of the calcareous sponges of our shores, the first of which (Grantia compressa) resembles little oval, flattened bags, which hang pendulous from rocks and weeds, sometimes solitary, but often in clusters. The smaller openings are thickly scattered over the flat sides of the bag, and the larger ones, through which the water is expelled, around the margin. When the sponge is out of the water and inactive, the two opposite sides of the bag are practically in contact, but, when active, the cavity is filled with water by means of the whip-cells that line it, and the sides of the sponge are then more or less convex.
Fig. 70.—Sycon ciliatum
The ciliated sycon (Sycon ciliatum), fig. 70, though of a very different appearance externally, is similar in structure to Grantia. It is also found in similar situations, and is not uncommon on many parts of the South Coast, from Weymouth westwards. The other example, Leucosolenia botryoides, shown in fig. 71, is a branching calcareous sponge, consisting of a number of tubes, all united to form one common cavity which is lined throughout with whip-cells. It is usually found attached to weeds.
Fig. 71.—Leucosolenia botryoides, with portion magnified
Nearly all our British sponges belong to the group Demospongia—common sponges; but the members of this group present a great variety of form and structure. Most of them have a skeleton consisting of siliceous spicules, but some have a horny skeleton, somewhat after the nature of that of the toilet sponges; and others, again, have fleshy bodies entirely, or almost entirely, unsupported by harder structures. They are sometimes known collectively as the Silicia, for the greater number of them have skeletons consisting exclusively of siliceous matter, while the so-called horny sponges usually have spicules of silica intermingled with the horny substance, and even those which are described as having no skeleton at all sometimes contain scattered spicules of silex.
Fig. 72.—Chalina oculata
As the spicules of sponges are in themselves beautiful objects, and are important to the naturalist, inasmuch as they form a basis for the classification of sponges, it is well to know by what means they may be separated from the animal for microscopic examination. The separation is based on the fact that nitric acid (aqua-fortis) will destroy organic matter while it has not the slightest action on silica. In some of our common horny sponges the fibres are so transparent that, when teased out and placed under the microscope, the siliceous spicules may be seen embedded within them, but the spicules, both in these and the fleshy sponges, may be separated completely from the animal matter by putting a fragment of the sponge in a test-tube, covering it with nitric acid, and boiling it for a short time. The tube should then be filled up with water and allowed to stand undisturbed for a time, after which the liquid is poured off gently from the sediment. If the sediment is then put under the microscope on a slip of glass, it will be seen to consist of grains of sand, of which there is always a considerable amount in the pores and cavities of a sponge, and the siliceous spicules.
Among the common objects of the sea shore is the horny skeleton of the sponge Chalina oculata, which is frequently washed on the beach by the waves, especially after storms. This sponge is not likely to be seen between the tide-marks except at the lowest spring tide, when it may be found suspended in a sheltered crevice or cave. The skeleton consists of a fine network of horny fibres, in the centre of which lie the spicules, imbedded in the horny material. The spicules are short and straight, tapering at both ends.
Fig. 73.—Halichondria panicea
The Bread-crumb sponge (Halichondria panicea) is even more common, for it is to be found on every rocky coast, encrusting weeds and rocks, often considerably above low-water mark. It is of a yellowish or pale greenish colour, and forms an incrustation varying in thickness from one-twentieth of an inch to half an inch or more; and, like most sponges, should be looked for in narrow crevices, under heavy growths of weeds, or in other situations where it is protected from the light. Sometimes its free surface is unbroken, except, of course, by the minute pores, and, here and there, the larger openings that serve for the outgoing currents; but when it is found encrusting a rock in patches of considerable size, the larger holes all occupy the summit of a little cone resembling a miniature volcano with its crater. This sponge is easily removed from the rock with the aid of a blunt broad-bladed knife, and retains its natural appearance to perfection if preserved in methylated spirit. Its horny skeleton is of a very compact nature, and the spicules are minute siliceous needles pointed at both ends.
Fig. 74.—Spicules of Halichondria, magnified
Rambling on the sea beach we frequently meet with old oyster and other shells perforated by a number of circular holes about the size of a pin’s head or less, and chalk and limestone rocks also are seen similarly bored. On breaking into or grinding down the substance we find that the openings are the ends of channels that form a network of canals and chambers, some of which are so near the surface that they are covered by an exceedingly thin layer of the calcareous substance. These canals and chambers form the home of the Boring Sponge (Cliona), which, although a very soft-bodied animal, has itself excavated them.
Fig. 75.—An Oyster Shell bored by Cliona
The manner in which the Cliona excavates such a complicated system of passages in so hard a material has naturally raised a considerable amount of curiosity, and those who have studied the matter are divided in opinion as to whether the work is done by chemical or by mechanical action.
Some of those who advocate the chemical theory suppose that an acid fluid is secreted by the sponge, and that the carbonate of lime forming the shell or stone is thereby dissolved; but such advocates have, as yet, failed to detect the presence of any acid substance in the body of the animal. Others ascribe the action to the solvent power of carbonic acid gas. This gas certainly has the power of dissolving carbonate of lime, as may be proved by a very simple experiment: Pour a little lime water into a glass, and blow into it through a glass tube. The lime water speedily becomes milky in appearance, the lime having been converted into particles of chalk or carbonate of lime by union with the carbonic acid gas from the lungs. Continue to blow into the liquid for some time, and the carbonate of lime will slowly disappear, being gradually dissolved by the excess of the gas—the gas over and above that required for the formation of the carbonate. Thus, it has been said, the carbonic acid gas evolved as a product of the respiration of the sponge is the agent by which the channels are excavated. Whatever be the acid to which this power is ascribed, whether it be the carbonic acid or a special acid fluid secreted for the purpose, there is still this difficulty in the way of accepting the theory, namely, that an acid, though it has the power of dissolving the mineral matter of a shell—the carbonate of lime—has no action on the laminæ of animal substance that form part of the structure. If we put the shell of a mollusc in hydrochloric or dilute nitric acid, we obtain, after the complete solution of the carbonate of lime, a substantial residue of animal matter which the acid does not touch, but in the case of Cliona both animal and mineral substances yield to its power.
Fig. 76.—Spicules of Cliona
Those who favour the mechanical theory assert that the material is worn away by siliceous particles developed by the sponge, and kept in constant motion as long as the animal lives; and the theory is supported by the statement that, in addition to the spicules of silica, which are pin-shaped, and occupy the interior of the animal, there are little siliceous granules scattered on the surface of the sponge which are kept in constant motion resembling that of cilia; and the minute particles of carbonate of lime that form a dusty deposit within the galleries are supposed to be the product of the rasping or drilling action of these granules.
The pin-shaped spicules of Cliona may be obtained for microscopic examination by breaking any old oyster shell that has formed its home, and brushing out the dust from the galleries; or, a part of the shell may be dissolved in acid, and the sediment examined for spicules on a slip of glass.
CHAPTER IX
THE CŒLENTERATES—JELLY-FISHES, ANEMONES, AND THEIR ALLIES
One of the most interesting groups of marine life is that including jelly-fishes and anemones. In it are the pretty little sea firs, so often mistaken for sea-weeds by the youthful admirers of these plants, who almost always include them in their collection of marine algæ; the transparent, bell-shaped jelly-fishes, which may often be seen in thousands during the summer, carried by the tides, and swimming gently by graceful contractions of their bells; and, most beautiful of all, the lovely anemones—the ‘sea flowers’ of the older naturalists, by whom they were regarded as forms of vegetable life.
Fig. 77.—Thread Cells of a Cœlenterate, magnified
1. Thread retracted 2. Thread protruded
The simplest animals of this group are minute jelly-like creatures, of a more or less cylindrical form, usually fixed at one end, and having a mouth at the other. The body is a simple hollow cylinder, the wall of which is made up of two distinct layers, while the cavity within serves the purpose of a stomach. The mouth is surrounded by a circle of arms or tentacles by means of which the creature is enabled to capture its prey. These arms are capable of free movement in every direction, and can be readily retracted when the animal is disturbed. They are also armed with minute oval, hollow cells, each of which has a slender filament coiled up into a spiral within its cavity. Each filament is capable of being suddenly protruded, thus becoming a free whip-like appendage, and these are so numerous as to be very effectual in seizing and holding the living beings on which the animal feeds. This would undoubtedly be the case even if they were capable of mechanical action only, but, in many instances at least, they seem to be aided by the presence of some violent irritant, judging from the rapidity with which the struggling prey is paralysed when seized, especially in the case of some of the larger members of the group.
Fig. 78.—The Squirrel’s-tail Sea Fir (Sertularia argentea), with a portion enlarged
The simple forms referred to increase by a process of budding, the buds appearing first as simple swellings on the side of the parent creature, and afterwards developing a mouth and tentacles, thus becoming exactly like the adult form. Clusters of eggs also are developed in the outer layer of the body-wall, and these are set free at intervals, and produce new individuals. These animals possess no blood system of any kind, and have no special organs for respiration, but the nutrient matter absorbed from the body-cavity permeates the soft structures of the flower-like body, and the oxygen required for respiratory purposes is readily absorbed from the surrounding water.
The higher cœlenterates differ in certain particulars from the lower forms just referred to. Thus, they frequently have a large number of tentacles around the mouth, often arranged in several distinct whorls. They have also a stomach separate from the general body-cavity, but communicating with the latter below; and the body-cavity is divided into compartments by a number of radiating partitions. Some, also, develop a hard, stony skeleton by secreting carbonate of lime obtained from the water in which they live.
Fig. 79.—Sertularia filicula
We often see, when collecting on the beaches of rocky coasts, and especially after storms, a number of vegetable-like growths, of a greyish or brownish colour, each consisting of one or more main stalks bearing a number of delicate branches. Some of them, by their peculiar mode of growth, have suggested the name of sea firs, and a few of these, together with other animals of the same group, may readily be recognised by the accompanying illustrations. They are the objects already referred to as being commonly included in collections of sea-weeds by young naturalists, but they are in reality the horny skeletons of colonies of cœlenterates of the simplest type, belonging to the division Hydrozoa.
Fig. 80.—Sertularia cupressina
If we examine them with a lens we find that there are little cup-like bodies projecting from each portion or branch of the stem-like structure, and that the stem itself is hollow, with a communicating pore at the base of each cup. This constitutes the skeleton only of the colony—the dead matter, so to speak, which persists after the living creatures have perished; but if the specimens collected have been obtained fresh from the sea, placed in a glass of sea water, and then examined with the aid of a lens, little jelly-like hydroids or polypites will be seen to protrude from the cups, and extend their short arms in search of food.
Fig. 81.—The Herring-bone Polype (Halecium halecinum)
Each of the little creatures has a tubular stalk which passes through the hole at the base of the cup, and is continuous with a tube of gelatinous material in the interior of the horny stem, and thus each member of the colony is directly connected with all the others, so that any nutrient matter collected and digested by one member may be absorbed into the central tube for the nourishment of the entire company of little socialists, the activity of the one being thus made to compensate for the laziness or incompetency of others. And this provision seems to be absolutely necessary for the well-being of the colony as a whole, for a close examination will often show that a kind of division of labour has been established, since it includes two or three distinct kinds of polypites, each adapted for the performance of a certain function. Thus, in addition to the feeding or nutritive members of the community, there are some mouthless individuals whose sole function seems to be the production of eggs for the propagation of the species, while others, also mouthless, develop an enormous number of stinging cells, probably for the protection of the whole community against its enemies, and these must therefore be provided, as we have seen they are, with a means by which they may derive nourishment through the agency of the feeding polypites.
Fig. 82.—Tubularia indivisa
Fig. 83.—The Bottle Brush
(Thuiaria thuja)
When the eggs are liberated from what we may call the reproductive members, they are carried away by the currents or tides, and soon develop into little larvæ which are very unlike the parent, since they are covered with minute vibratile cilia by means of which they can swim freely. This they do for a period, and then settle down, lose their cilia, become stalked, and thus constitute the foundation of a new colony. A tubular stalk grows upward from its root, new members are added as outgrowths or buds from their progenitor, and so the growth proceeds until an extensive colony of hundreds of individuals has been formed.
We have spoken of the hydroid communities as being washed up on the beaches of our rocky coasts, but the collector of these interesting objects should not depend on such specimens for purposes of study. It is undoubtedly true that splendid examples of the sea firs and their allies are frequently washed up by the waves, including some species that inhabit deep water, and which are, consequently, not to be found by the ordinary collector in their proper habitat, and that these may often be secured with the polypites still alive; but several species are to be obtained between the tide-marks, especially at extreme low water, growing on rocks, weeds, and shells; and we have often met with good specimens, still alive, attached to the shells of whelks, scallops, &c., in fishmongers’ stores, even in inland towns.
Fig. 84.—Antennularia antennia
Sometimes individual polypites become detached from a colony, and develop into little umbrella-shaped jelly-fishes, about a fifth of an inch in diameter; and these float about freely, keeping themselves near the surface by rhythmic contractions of their ‘bells,’ the margins of which are fringed by numerous fine tentacles. The mouth is situated centrally on the under side, and is surrounded by a circular canal from which proceed radiating tubes; and pigmented spots, supposed to be rudimentary eyes, are formed round the edge. These little bodies are called Medusoids, and may frequently be seen floating round our coasts towards the end of the summer. In the water they are almost invisible on account of the extreme transparency of their bodies; but if a muslin net be drawn through the water from the stern of a boat, and the net then gently turned inside out in a vessel of sea water, a number of medusoids may be obtained for examination. These creatures produce eggs which yield small ciliated larvæ that swim about freely for a time, and then settle down and establish stalked colonies as previously described.
The larger jelly-fishes or Medusæ so frequently seen floating in enormous numbers near the surface of the sea during the summer months are allied to the medusoids. Their bodies are so soft that it is a difficult matter to remove them from the water without injury, and when removed their graceful forms are completely destroyed by the pressure of their own weight. When left stranded on the beach, as is often the case, they seem to dissolve almost completely away, so readily does the soft animal tissue disintegrate in the large proportion of water, which forms about 95 per cent. of the weight of the whole body.
Those who desire to examine the nature and movements of the medusæ will find it necessary to observe them in water. The creatures may be lifted out of the sea in a vessel placed below them, and then transferred to a glass tank or a still rock pool by submerging the vessel and allowing them to float out. It will then be observed that the mouth is situated at the summit of a tube that projects from the middle of the under side of the ‘bell,’ and is surrounded by lobed or frilled lips. Marginal tentacles also generally fringe the edge of the bell, projecting downwards into the water. Round the circumference of the body may be seen a circular canal, from which several tubes converge towards, and communicate with, the cavity of the stomach.
When a medusa is inactive, its body gradually sinks to the bottom, being usually slightly heavier than the water in which it lives; but it is enabled to keep afloat by those rhythmic contractions of the bell with which we are so familiar. It seems that the medusæ are very sensitive to various external conditions, for they frequently disappear simultaneously from the surface water, and as suddenly reappear in shoals when the conditions are more favourable; but it is difficult to understand the causes which give rise to these remarkable movements.
The medusæ are often termed the Acalephæ—a word which signifies ‘nettles,’ and they are popularly known as sea nettles. They all possess stinging cells, which are distributed most thickly in the tentacles, and some of the larger species are undoubtedly able to produce an impression on the bodies of unwary bathers, while almost all have the power of paralysing the living prey on which they feed.
By far the commonest of the jelly-fishes of our seas is the beautiful blue medusa—Aurelia aurita. This species appears in enormous shoals during the summer, and large numbers are washed upon flat, sandy beaches. They vary in size from two or three inches to nearly a foot in diameter, and may be recognised from our illustration. The ‘bell’ is umbrella-shaped, and is so transparent that the stomach with its radiating canals may be seen through its substance. Around the margin there are little pigment spots which are supposed to be rudimentary eyes, and little cavities, containing a clear fluid, that are thought to serve the purpose of ears.
Fig. 85.—Aurelia aurita
On the under surface may be seen the square mouth, furnished with four long and graceful frilled lips, which are richly supplied with stinging cells; also the four ovaries or egg-producing organs, rendered conspicuous by their violet colouring.
Fig. 86.—The early Stages of Aurelia
The life history of Aurelia is most interesting. The eggs are produced in pouches that communicate directly with the stomach-cavity, and these give rise to little ciliated larvæ that are ejected through the mouth, and then swim about freely in the water for a time. After this they settle at the bottom, lose their cilia, and become little cylindrical jelly-fishes, fixed by a short stalk-like foot to rocks or weeds Numerous tentacles develop as the creatures increase in size, and a number of transverse furrows appear at the surface. The furrows gradually increase in depth until, at last, the body is broken up into several star-like discs, each of which floats away and develops into a new medusa.
Other jelly-fishes, some of which are considerably larger than Aurelia, frequent our seas, and are often to be seen stranded on the beach. Two of these—Rhizostoma and Chrysaora—are figured. Although they differ considerably in form from the blue aurelia, they closely resemble it in general structure and habits.
Fig. 87.—Rhizostoma
Fig. 88.—Chrysaora
When strolling on flat, sandy beaches, especially in the spring and early summer, we commonly see what appear to be little balls of exceedingly transparent and glassy jelly, no larger than an ordinary marble. If picked up and examined, we observe that they are not quite spherical, but oval in form, with a little tubercle at one end, and eight equidistant bands running from this to the opposite end, like the meridians on a globe.
This extremely beautiful little creature is one of the cœlenterates, belonging to the division Ctenophora, or comb-bearing jelly-fishes, so called because they possess comb-like ciliated plates, and is called the Globular Beroe (Cydippe pileus).
The ctenophores are very active creatures, swimming freely in the open seas by means of their numerous cilia; and, although of such delicate structure, are very predaceous, devouring small crustaceans and other marine animals. They are usually globular in form, but some are like long ribbons, and almost all are remarkable for their wonderful transparency, which renders them nearly invisible when floating in water. They have not the power of stinging or paralysing their prey, as the medusæ have, but their fringed arms are provided with adhesive cells by which they hold their prey tenaciously.
Fig. 89.—Cydippe pileus
In order to observe the form and habits of the Beroe we transfer it to a vessel of sea water, when it immediately displays its regular spheroid form, and its eight rows of comb-like plates which form the meridians before alluded to. Its mouth is situated on the little tubercle at what we may call the lower pole, for it is the habit of the Beroe to swim in an inverted position, and the digestive cavity may be seen through its glassy body.
At first no appendages of any kind are visible, but soon the animal protrudes two long and exceedingly slender arms, fringed with slender gelatinous threads, from two cavities, at opposite sides of the body, into which they can be withdrawn. A close examination will also reveal the rapid movements of the cilia of its combs, and it is remarkable that these do not always work together, the animal being able to move any of its plates independently, and to reverse their motion when occasion requires. It has no tentacles corresponding with those of jelly-fishes and anemones, but is assisted in the capture of its prey by its two long arms, the chief use of which, however, seems to be that of a rudder for steering.
If the Beroe is left out of water for some time, the water which forms such a large proportion of its body evaporates, leaving an almost imperceptible residue of solid matter; and if left in water after it is dead, its substance rapidly dissolves away, leaving not the slightest trace of its presence. There seems to be no satisfactory way of preserving this beautiful form of animal life. If placed in strong spirit the water is rapidly extracted from its body, and its animal substance shrivelled to a minute, shapeless mass; while in weak spirit and in other fluid preservatives it becomes more or less distorted, and deprived of its beautiful transparency, or else it disappears altogether.
We now come to the great favourites among the cœlenterates—the beautiful anemones-the animated flowers of the ocean, remarkable not only for their lovely flower-like forms, but also for the great variety of colour and of habits which they display. These, together with the corals, form the division of the cœlenterates known as the Zoantharia, characterised by the possession of simple tentacles, the number of which is a multiple of either five or six. The latter differ from the former mainly in the power of secreting a calcareous skeleton which remains attached by its base after the animal substance has decayed.
The expanded anemone exhibits a more or less cylindrical body, attached by a suctorial base to a rock or some other object, and a broad circular disc above. In the centre of this disc is the mouth, surrounded by the tentacles, often very numerous, and arranged in one or more whorls. When the animal is inactive the tentacles are usually completely withdrawn, and the body contracted into a semiglobular or pear-shaped mass which is very firm to the touch.
The general internal structure of an anemone may be made out by simple dissections, and the examination conducted with the specimen submerged in water. A longitudinal section will show that the body is a double tube, the outer being formed by the body-wall, and the inner by the wall of the stomach. Thus there is a body-cavity distinct from that of the stomach, but the two will be seen to communicate below, since the stomach-wall does not extend as far down as the base. It will be seen, too, that the body-wall is made up of two distinct layers—an outer one, that is continued inward at the mouth to form the inner wall of the stomach, and an inner one that lines the whole of the body-cavity. The latter contains the muscular elements that enable the anemone to contract its body.
When the animal is expanded, the whole interior is filled with sea water, as are also the tentacles, which are hollow tubes, really extensions of the body-cavity, and formed by prolongations of the same two layers that constitute the body-wall. As it contracts this water is expelled, partly through the mouth, and partly through small openings that exist at the tips of the tentacles.
Fig. 90.—Section of an Anemone
t, tentacles; m, mouth; s, stomach; b c, body-cavity p, mesentery; o, egg-producing organ
The outer layer of the body-wall is provided with stinging cells which serve not only to protect the anemone from its enemies, but also to aid it in the capture of its prey, for which latter purpose they are distributed in much greater abundance in the tentacles.
The body-cavity is divided into a number of communicating compartments by means of vertical partitions running from the body-wall and converging towards the centre of the cavity. These are called mesenteries, and are extensions of the inner layer of the body-wall. Five or six of these are larger than the others, extending from disc to base, and are called primary mesenteries. Between these are an equal number of smaller secondary mesenteries; and, sometimes, a third set of still smaller tertiary mesenteries.
These internal partitions are best displayed in a transverse section of the body, which shows the double tube formed by the walls of the body and the stomach, together with the wheel-like arrangement of the mesenteries. At one time all animals that had a radial symmetry—the regular arrangement of parts round a common centre—were grouped together under the title of Radiata; but it has since been recognised that the creatures of this group exhibited such a great diversity of structure that they have been re-classified into two main divisions, one of which constitutes the cœlenterates which we are at present considering, and the other containing such creatures as star fishes and sea urchins.
Fig. 91.—Stinging Cells of Anemone, highly magnified
a and c, with thread protruded; b, with cell retracted
Fig. 92.—Diagrammatic transverse section of an Anemone
S, stomach; bc, body-cavity; m′, m″, m‴, primary, secondary, and tertiary mesenteries
Fig. 93.—Larva of Anemone
On the surface of the mesenteries of the anemone may be seen the ovaries or egg-producing organs. These discharge the ova into the general body-cavity, after which they are ejected through the mouth. The embryos are minute jelly-like creatures that have an active existence, swimming about freely in the ocean by means of vibrating cilia, but after this period of activity they settle down and fix themselves, gradually assuming the adult form common to the species.
The habits of sea anemones are particularly interesting, and it will well repay anyone to make a study of these animals in their natural haunts as well as in the aquarium. The gentle swinging of the tentacles when searching for food, the capture and disposal of the prey, the peculiar modes of locomotion, and the development of the young, are among the chief points of interest. As regards locomotion, the usual method of moving from place to place is by an exceedingly slow gliding of the base or ‘foot’; and while some anemones are almost constantly on the move, others hardly ever stir from the secluded niche in which they have taken up their abode.
Sometimes an anemone will detach itself from the rock, and drag itself along, but very slowly, by means of its tentacles, sometimes inverting its body and walking on its head, as it were, and though one may never have the opportunity of witnessing this manœuvre on the shore, we have found it far from an uncommon occurrence in the aquarium.
The natural food of anemones consists of small crustaceans, such as shrimps, and crabs, molluscs, small fishes, and in fact almost every kind of animal diet, and there need never be any difficulty in finding suitable viands for species kept in captivity. It is really astonishing to see what large morsels they can dispose of with the assistance of their extensile mouths and stomachs. It is not even necessary, indeed, that the morsel be so small as to be entirely enclosed by the walls of its digestive cavity, for the anemone will digest one portion while the other remains projecting beyond its mouth. Further, it will even attack bodies which it cannot swallow at all, by protruding its stomach so as to partially envelope them, and then digesting the portion enclosed. Indigestible portions of its food, such as the shells of small molluscs, are ejected through the mouth after the process of digestion has been completed.
We have already referred to the reproduction of sea anemones by means of eggs, but it is interesting to note that they may also increase by a division of the body into two or more parts, and that this division may be either natural or artificial.
If an anemone be cut into halves longitudinally, each half will develop into a complete animal. If cut transversely, the upper portion will almost always develop a new suctorial disc, and produce a new individual complete in every respect; and it has been stated that the basal portion of the divided animal will also, occasionally, produce a new disc and tentacles.
The natural division of the anemone has frequently been spoken of as by no means an uncommon occurrence, but, as far as our experience of captive anemones go, this mode of multiplication does not seem to take place except as the result of some mechanical force applied, or as a means by which the animal may relieve itself of a solid body that it is unable to eject. Thus, on one occasion, when a stone had slipped so that its narrow edge rested across the middle of the disc of a large Mesembryanthemum, the animal, apparently unable to free itself from the burden, simply withdrew its tentacles and awaited results. In a few days two individuals were to be seen, one on either side of the stone, both undoubtedly produced as the result of the pressure applied. This instance seems to be exactly akin to artificial division, for it is far more likely that the animal was severed by the simple pressure of the stone than that it divided itself to be relieved of its burden.
On another occasion an anemone that had almost entirely surrounded a mussel on which it had been feeding, gradually released itself of the shell by a longitudinal division of its body; but here, again, it is probable that the fission was the result of pressure applied rather than of any power on the part of the animal.
A few of the British sea anemones are shown on Plates [II.] and [III.], and although the coloured illustrations will probably suffice for purposes of identification, yet a short description of each one represented may be acceptable.
The most common and most widely distributed species is undoubtedly the familiar Beadlet (Actinia mesembryanthemum—Plate II., figs. 1, 2, 3), which is to be found on every bit of rocky coast around the British Isles, and even on some stony beaches where there are no standing rocks between the tide-marks.
The colour of this species is exceedingly variable, but the most abundant variety is of a liver-brown colour, with crimson disc and tentacles, brilliant blue spots round the margin of the disc, and a line of bright blue around the base. In others the prevailing colour is deep crimson, orange, yellowish brown, or green. Fig. 1 represents a variety commonly known as the Strawberry Beadlet (Fragacea), which is distinguished by its superior size, and in which the dark-red ground is often conspicuously spotted with green.
Two members of the same genus are also shown on Plate [III]. One of these—A. glauca (fig. 3)—is of a bluish-green colour; while the other—A. chiococca (fig. 4)—is bright scarlet, with deep crimson disc and white spots round the disc.
Plate II
SEA ANEMONES
| 1, 2, 3, Actinia mesembryanthemum. | 6. Sagartia bellis. |
| 4. Caryophyllia Smithii. | 7. Balanophyllia regia. |
| 5. Tealia crassicornis. | 8. Actinoloba dianthus. |
The general form of this genus is that of an expanded flower on a short column; the name Beadlet is applied on account of the little bead-like projections on the margin of the disc. The tentacles number nearly two hundred in a fully grown individual, and are arranged in several rows; but when the animal is disturbed and the tentacles retracted, its form is almost hemispherical.
It is interesting to note that A. mesembryanthemum not only exists in varieties distinguished by distinct colours, but that the same individual will sometimes change its tint, as may be observed when it is kept in the aquarium; and it may be mentioned, by the way, that it is very easily reared in captivity, either in the natural or the artificial salt water, for not only may the same individuals be kept alive for years with only a moderate amount of attention, but their offspring may be reared without difficulty.
On [Plate II.] (fig. 8) are two illustrations of the beautiful Actinoloba dianthus, which grows to a length of five or six inches, and is easily distinguished by its expanded and frilled disc, its very numerous short and slender tentacles, and its tall, pillar-like body. Its colour is somewhat variable, being either salmon, flesh-colour, cream, white, red, orange, or brownish; but whatever be the tint of the body and tentacles, the margin of the mouth is always red or orange. When young it may easily be mistaken for another species, as its disc is not then frilled, and the tentacles are much fewer in number.
This pretty anemone usually inhabits deep water, and is frequently brought in, attached to shells and stones, by trawlers, but it may be commonly observed in the dark crevices of rocks, a little above low-water mark, where it is usually seen contracted into a ball, or even so much flattened that it looks like a mere pulpy incrustation of the rock. It is very common on the rocky coasts of Dorset, Devon, and Cornwall, as well as in many parts of Scotland and Ireland.
Like the Beadlet, it is easily kept alive in the aquarium, where it commonly multiplies by natural division; but as it does not generally expand in full daylight, its beauty is often better observed at night by artificial light.
On [Plate II.] (fig. 5) we have an illustration of the beautiful Dahlia Wartlet (Tealia crassicornis), which may be readily recognised by its thick, banded, horn-like tentacles, and the numerous little adhesive warts that almost cover the surface of its body.
This species is as abundant as it is beautiful, for it is to be found in plenty on almost every rocky coast, where it may be seen in the rock pools and in the crevices of rocks near low-water mark. The diameter of its cylindrical body often reaches two or three inches, while the expanded tentacles embrace a circle of four or five inches. Specimens even much larger than this are sometimes obtained by dredging in deep water.
Fig. 94.—The Trumpet Anemone (Aiptasia Couchii), Cornwall; deep water
The ‘Dahlia’ is not so frequently seen by sea-side collectors as its abundance would lead one to expect, and this is principally due to the fact that it not only conceals itself in narrow and out-of-the-way crevices and angles of rocks, but also that, on the retreat of the tide, it generally covers itself with small stones, fragments of shells, &c., held fast to its body by means of its numerous suckers. In this manner it conceals its beauty so well that the sense of of sight, is necessary in determining its whereabouts. As a rule, however, it does not resort to this method of concealment when it inhabits deep water, or even a permanent rock pool between the tide-marks, and thus it is in the latter home where one may expect to see this sea flower in all its glory, for when permanently covered with water it will seldom hide its crown, except when alarmed, or when in the act of swallowing its food.
Fig. 95.—Peachia hastata, S. Devon
It should be noted, too, that the rock pool is the right place in which to study the habits of this anemone, for it is not nearly so easy to rear in the artificial aquarium as the species previously described, and, moreover, it requires a great deal of food. We have found it live longest in running water, kept cool, and frequently renewed by supplies fresh from the sea. It may be fed on almost any, if not every, form of animal life inhabiting a rock pool. A small fish or a prawn is perfectly helpless when once it is seized by the creature’s tentacles. Mussels, winkles, limpets, &c., are eagerly swallowed, and the indigestible shells disgorged after the animal substance has been dissolved by the digestive fluid. Even the active shore crab, armed as it is with a coat of mail and powerful pincers, is no match for its powerfully adhesive tentacles; nor do the sharp spines of the prickly urchin preserve it from so voracious a creature.
The rocky coasts of Devon and Cornwall are the chief haunts of the pretty ‘Daisy Anemone’ (Sagartia bellis), and here it is very abundant in places. This species lives in holes and crevices of the rocks, its body usually entirely hidden from view, but its dark brown disc, intersected by bright red radiating lines, and fringed with numerous small tentacles, fully exposed to view as long as it is submerged. The length of its body is always adapted to the depth of the hole or crevice in which the animal lives, and may vary from half an inch to two or three inches, the diameter of the columns being greatest where the length is least.
Fig. 96.—Sagartia pallida, Devon and Cornwall
Sometimes the ‘Daisy’ may be seen living a solitary life, having settled down in a hole just large enough to accommodate it, but more commonly it is seen in company with several others of its species, occupying a crevice in a rock pool, and often so closely packed together that the tentacles of each individual are intermingled with those of its neighbours, thus exhibiting a more or less continuous cluster or line of ‘flowers,’ each disc being from one to two or three inches in diameter when fully expanded.
On account of the peculiar positions selected by this species, it is not easily removed without injury, and hammer and chisel are almost always necessary for its removal; but if it is obtained without injury, and transferred to the indoor aquarium, but little difficulty will be found in keeping it alive and in health. It is also very prolific, and a single specimen placed in the indoor tank will frequently produce a large number of young.
The colour of S. bellis, like that of many of our anemones, is very variable, but the species may easily be recognised by the radiating lines of the disc, and the numerous small tentacles. One variety, however, deviates considerably in form, colour, and habit from the normal. It (Plate II., fig. 6) is of a dull yellow colour, and has a much less graceful form; and, instead of living in the holes and crevices of rocky coasts, where it would be washed by fresh sea water at every tide, it inhabits the muddy and fœtid waters of narrow inlets of the sea in the neighbourhood of Weymouth.
Fig. 97.—Sagartia nivea, Devon and Cornwall
Three other species of the same genus are represented on [Plate III]. The first of these—Sagartia troglodytes, sometimes called the Cave-dweller (fig. 1)—though very variable in colour, may be known by its barred tentacles, each with a black B-like mark near its base. It lives in sheltered, sandy, or muddy hollows between the rocks on most rugged coasts, often with its body entirely buried beneath the sediment; or, if only partially buried, the projecting portion of the column concealed by particles that adhere to its suckers.
The column is usually of an olive colour, striped longitudinally with a paler tint, and sometimes reaches a length of two inches, while the diameter of the expanded ‘flower’ may even exceed this length.
This anemone is not a very conspicuous object of the shore, since the exposed portion of its column is usually more or less covered by sedimentary matter, and the tentacles are generally of a tint closely resembling that of the surrounding surface. Thus the anemone is protected from its enemies by its peculiar habit and colouring, while at the same time the spreading tentacles constitute an unseen but deadly snare for the unwary victims that come within their range.
Fig. 98.—Corynactus viridis, Devon and Cornwall
This species is often difficult to secure without injury on account of its preference for narrow chinks in awkward situations, but we have found that it is sometimes easily removed by first clearing away the surrounding débris, and then gently pushing it from its hold by means of the finger-nail. It seems, in fact, that its base is occasionally quite free from the underlying rock, being simply imbedded in sand or mud. In other cases hammer and chisel are necessary to remove it from its snug hole.
If placed in the aquarium it should be allowed to get a foot-hold in a suitable hole or crevice, which should be afterwards partially filled with sand. It is not difficult to keep, and although not a showy species, and having a decided preference for shady places, yet its habits will be found interesting.
The Orange-disked Anemone (Sagartia venusta) is represented in fig. 2 of the same plate. It may be easily distinguished by its brilliant orange-coloured disc, surrounded by white tentacles, which, when fully expanded, commands a circle of from one to one and a half inches. South-west Wales is said to be the headquarters of this pretty sea flower, but we have found it abundant on parts of the north Devon coast, especially in places between Ilfracombe and Lynton. Like the last species, it may be termed a cave-dweller, for it delights to hide in corners and crevices that are so overhung with rocks and weeds that the light is never strong.
Yet another species of this genus (S. rosea) is depicted in [Plate III.], fig. 8. It has been termed the Rosy Anemone, from the brilliant rosy tint of its numerous tentacles. The column is generally of a dull brown colour, with suckers scattered over the upper portion, and the flower reaches a diameter of an inch or more. This anemone may be seen at rest on overhanging rocks near low-water mark when the tide is out, its disc only partially hidden, and the tips of its bright tentacles just exposed. It may be seen on many parts of the Devon coast, and is, or, at least, was, abundant in localities near Brixham and Shaldon.
On the same plate is an illustration (fig. 7) of one of the most abundant and most interesting of our anemones. It is commonly known as the Opelet, and its scientific name is Anthea cereus. Almost everyone who has done a little collecting on the rocky shores of the south-west of England, or on the shores of Scotland or Ireland, must have seen this species, easily distinguished by its long, slender, smooth tentacles, all of about equal length, and presenting a waxy appearance. These appendages are usually green and tipped with pink, but sometimes pale yellow or red, and are of such a length that they cover a circle of five or six inches.
This species is decidedly of social disposition, for a number may generally be seen in a cluster, crowded closely together; and when we see them, as we often do, occupying a little tide pool that contains scarcely sufficient water to enable them to give free play to their tentacles, and exposed for hours to the full blaze of the summer sun, we naturally form the opinion that they ought to require no special care in the indoor aquarium. And this is actually the case, for they thrive well with but little trouble.
Perhaps the chief interest attached to this anemone is the deadly nature of its grip. The numerous long tentacles have considerable clinging power throughout their length, and their paralysing power is very considerable compared with that of many other species of the same size. Even the human skin is more or less affected by the irritating influence of this species, a sensation approaching to a sting being sometimes produced, and the skin showing visible signs of the injury done. The grip, too, is so tenacious that tentacles are sometimes torn off when the hand is quickly withdrawn from their hold.
Our next example is the Red-specked Pimplet (Bunodes Ballii), shown in fig. 5 of [Plate III.], which has received its popular name on account of the numerous longitudinal rows of red-specked warts that run down its short yellow column, and other red spots on the column itself, between the rows. Its tentacles are usually pale yellow or white, but sometimes grey or greenish, and often tinged with pink.
Fig. 99.—Bunodes thallia, West Coast
This anemone is common on some parts of the coasts of Hampshire, Dorset, Devon, and Cornwall, as well as on the south coast of the Isle of Wight, and may be found in secluded crevices of the rocks, or under the large stones that are scattered on the beach.
Plate III
SEA ANEMONES
| 1. Sagartia troglodytes | 5. Bunodes Ballii |
| 2. Sagartia venusta | 6. Bunodes gemmacea |
| 3. Actinia glauca | 7. Anthea cereus |
| 4. Actinia chiococca | 8. Sagartia rosea |
The Gem Pimplet (Bunodes gemmacea) is shown on the same plate (fig. 6). It is easily distinguished by the six conspicuous longitudinal rows of large white warts, between which are several other rows of smaller ones. The column is pink or brownish, and the thick tentacles are conspicuously marked by light-coloured roundish spots. It is not uncommon on the south-west coast of England, where it may be seen in the rock pools and on the surfaces of rocks between the tide-marks. Both of the species of Bunodes above mentioned may be kept in the aquarium without much trouble.
All the anemones so far briefly described are quite devoid of any kind of skeleton, the whole body being of a pulpy or leathery consistence, but some of our British species develop an internal calcareous skeleton, consisting of a hollow cylinder of carbonate of lime secreted by the body-wall, and attached to the rock by means of a similar deposit formed in the base, and also, within the cylinder, of a number of thin plates attached to the skeleton of the body-wall and projecting inwards towards the axis, thus resembling, in fact, the skeletons of a number of the tropical corals with which we are familiar. The animals in question are often collectively spoken of as British corals.
Fig. 100.—Bunodes gemmacea, with tentacles retracted
One of the finest of these corals is the Devon Cup-Coral (Caryophyllia Smithii), figured on [Plate II]. It may be found in many parts of Devon and Cornwall, attached to the rocks between the tide-marks, often in very exposed places, but is much more abundant in deep water.
Its skeleton is white or pale pink, and very hard, and is in itself a beautiful object. The animal surrounding this stony structure is of a pale fawn colour, with a white disc relieved by a deep brown circle round the mouth. The tentacles are conical, almost colourless and transparent, with the exception of the deep-brown warts scattered irregularly over them, and are tipped by rounded white heads.
Of course a hammer and chisel are necessary for the removal of these corals, but they are hardy creatures, and may be kept for a considerable time in captivity. Their habits, too, are particularly interesting, and two or more may sometimes be found with skeletons attached, suggesting that branched arrangement so common in many of the corals from warmer seas.
Another of these stony corals (Balanophyllia regia) is shown on the same plate. It is much smaller than the last species, but exceedingly pretty. It is also much less abundant, being confined almost exclusively to the coast of North Devon, and is seldom seen far above the lowest ebb of the tide.
Fig. 101.—Caryophyllia cyathus
Our few brief descriptions of British anemones and corals have been confined to those species which appear in our coloured plates, but we have interspersed here and there between the text a few illustrations which will assist in the identification of other species and also help to show what a rich variety of form is exhibited by these beautiful creatures. Some of these inhabit deep water only and are consequently beyond the reach of most sea-side observers during the ordinary course of their work; yet they may often be seen in fishing villages, especially in the south-west, where they are frequently brought in among the haul of the trawlers, attached either to shells or stones; and live specimens of these deep-sea anemones may even be seen on the shells of whelks and bivalve molluscs in the fishdealers’ shops of London and other large towns.
Fig. 102.—Sagartia parasitica
One of the species in question—the Parasitic Anemone (Sagartia parasitica) is generally found on the shell of the whelk or some other univalve; and, if removed from its chosen spot, it will again transfer itself to a similar shell when an opportunity occurs. This interesting anemone is usually seen among the dredgings of the trawler, but may be occasionally met with on the rocky coasts of the south-west, at extreme low-water mark. Though sometimes seen attached to stones, shells may undoubtedly be regarded as constituting the natural home of the species, and many regard the former position as accidental or merely temporary, and denoting that the animal had been disturbed and removed from its favourite spot, or that circumstances had recently rendered a change of lodgings necessary or desirable. Further, the shell selected by this anemone is almost always one that is inhabited by a hermit crab; and this is so generally the case that the occasional exceptions to the rule probably point to instances in which the occupant of the shell had been roughly ejected during the dredging operations.
Fig. 103.—The Cloak Anemone (Adamsia palliata) on a Whelk Shell, with Hermit Crab
The peculiar habit of the anemone just referred to makes it an interesting pet for the aquarium, for if removed from its natural home, and placed in the aquarium with a hermit crab, it will, sooner or later, as the opportunity occurs, glide from its hole on the stone or rock, and transfer itself to its favourite moving home.
It may be difficult at first to see what advantage can accrue to the anemone by the selection of such a situation; and, moreover, it becomes an interesting question as to whether the advantage is a mutual one. Close observations may, and already have, thrown some light on this matter, though it is probable that there still remains something to be learnt concerning the relations which exist between the inside and outside occupants of the portable house.
It may be noticed that the anemone almost invariably takes up a position on the same portion of the shell, and that, when fully expanded, its mouth is usually turned towards that of the crab. This seems to be a very favourable position for the anemone, since it is one that will enable it to catch the waste morsels from the crab’s jaws by its expanded tentacles. But it is, perhaps, not so easy to suggest a means by which the anemone can make an adequate return for free board thus obtained. It is well to remember, however, that crabs are regarded as such delicate morsels by fishes that we have already spoken of the value of these crustaceans as bait; while the fact that sea anemones remain perfectly unmolested in rock pools inhabited by most voracious fishes, coupled with the fisherman’s experience as to the absolute worthlessness of anemones as bait, is sufficient in itself to justify the conclusion that these creatures are very distasteful to fishes. This being the case, it is possible that the hermit crab is amply repaid by the anemone for its liberal board not only by partially hiding the crab from the view of its enemies, and thereby rendering it less conspicuous, but also by associating its own distasteful substance with that which would otherwise be eagerly devoured.
When the hermit grows too large to live comfortably in its shell, a change of home becomes necessary, and it is interesting to observe that the anemone living on the outside of the shell transfers itself at the same time; and this is a matter of vital importance to the crab, since it usually changes its lodging at the moulting period, at which time its body is covered by a soft skin, and is then even more acceptable as prey to the fishes. Thus the anemone accompanies its host, affording it continued protection during the period of its greatest danger.
Before leaving the cœlenterates we must refer to one other form which, though not often having its habitat between the tide-marks, is nevertheless a very common object in the neighbourhood of fishing villages, where the refuse from the nets used in deep water has been thrown on the beach. We refer to the peculiar animal known to fishermen as ‘Dead Men’s Fingers,’ and to the naturalist as the Alcyonium.
When seen out of water it is not by any means an inviting object, but is apparently a mass of gristly matter, of a dirty yellowish or brownish colour, sometimes flattened and shapeless, and sometimes lobed in such a manner as to suggest the popular name so commonly applied. It is always attached to some hard object, such as a stone or a shell, and is so frequently associated with oyster shells that it is by no means an uncommon object in the fishmonger’s shop, from which we have often obtained live specimens for the aquarium.
When placed in sea water it gradually imbibes the fluid surrounding it, becoming much swollen. Then little star-like openings appear, the circumference of each of which protrudes so as to form a little projecting tube. Finally, a crown of eight little tentacles is protruded, and the mass, so uninteresting at first sight, reveals itself as a colony of pretty polyps.
In general structure the Alcyonium resembles the sea anemone, but the firm body-wall of the colony is supported and protected to some extent by the presence of minute spicules of carbonate of lime; and it is interesting to note that while the tentacles of anemones and corals make up a number that is a multiple of either five or six, those of the Alcyonaria and the allied ‘Sea pens’ are always in multiples of four.
CHAPTER X
STARFISHES, SEA URCHINS, ETC.
Still passing up the scale of animal life, we now come to the Echinodermata—the other sub-kingdom which we have already referred to as forming, with the Cœlenterates, the old division of Radiata. The term Echinoderm signifies ‘hedgehog skin,’ and is applied to the group on account of the fact that the majority of its species possess a skin that is either distinctly spiny, or exhibits numerous more or less defined prominences. This skin is also supported and hardened by the deposit of little plates or spicules of carbonate of lime, all joined together so as to form a kind of scaffolding or ‘test’ for the protection of the animal; and this secretion of carbonate of lime is not always confined to the outer skin, for, in some cases, it occurs in the walls of the internal organs as well.
Most of the animals of this sub-kingdom display a regular radiate symmetry; that is, the parts of their bodies are arranged regularly round a common axis, and the arrangement is usually a five-fold one, as may be observed in the case of the common Five-fingered Starfish of our coasts (see [Plate IV.]), and it is worthy of note that this radiate disposition of parts is not merely external, but that, as in the case of anemones and jelly-fishes, it also obtains within, and determines the arrangement of the internal organs. Further, although this radiate symmetry characterises the adult animals of the group we are considering, yet some show a tendency towards bilateral symmetry (parts arranged equally on two opposite sides of a common axis), while this is the rule, rather than the exception, with the early stages or larvæ of these creatures. Observe, for instance, the larva of the common Brittle Starfish, the adult of which species exhibits an almost perfect radiate symmetry, and we see something more than a mere trace of a two-sided disposition.
We have not to look far into the structure of any typical echinoderm to see that it is a distinct advance on the anemones in the matter of organisation. To begin with its digestive system—this consists of a tube having no communication with the general body-cavity, but remaining quite distinct throughout its length, with both ends communicating directly with the exterior. Its nervous system also is more highly developed, for it has a well-formed ring of nerve matter round the mouth, from which pass two or three systems of nerve fibres, each system having its own special function to perform. The sense organs, however, do not appear to be well developed, though there exist certain ‘pigment spots,’ in which nerve fibres terminate, and which are supposed to serve the purpose of eyes.
Fig. 104.—Larva of the Brittle Starfish
One of the most interesting features in connection with the echinoderms is undoubtedly the structure and function of the apparatus for locomotion. Examine a live sea urchin, or the common five-rayed starfish, in a rock pool or aquarium, and it will be seen to possess a large number of soft, flexible, and protrusible processes, each of which terminates in a little sucking-disc that enables the animal to obtain a good ‘foot-hold;’ and, having fixed itself on one side by means of a number of these little ‘feet,’ it is enabled, by the contraction of certain muscles, to pull itself along.
The little feet we are examining are really tubes filled with water, and capable of being inflated by the injection of water into them from within the body of the animal. Each one communicates with a water tube, several of which (usually five) radiate from a circular canal of water that surrounds the mouth. This circular canal does not communicate with the mouth, but with a tube, known as the ‘stone canal’ because of the carbonate of lime deposited within its walls, that opens at the surface of the body on the opposite side, and is guarded at the orifice by one or more perforated plates through which water gains admission. Thus the animal can fill its ‘water system’ direct from the sea, and, by the contraction of muscles that surround the main canals, force this water into the little ‘tube-feet,’ causing them to protrude and present their sucking-discs to any solid object over which it desires to creep. We may observe, however, that some of the little protrusible tubes have no sucking-discs, and probably serve the purpose of feelers only; also, that while these tube-feet are the principal means of locomotion in certain species, in others the movements of the body are performed almost exclusively by the five or more rays that extend from the centre of the animal, and which are readily curved into any desired position by the action of well-developed muscles.
All the echinoderms come within the domain of the marine naturalist, for no members of the sub-kingdom are inhabitants of fresh water; and it is interesting to observe that, unlike the animals previously described, none of them live in colonies.
A general examination of the various starfishes to be found in our seas will show that they may be divided into three distinct groups. One of these contains the pretty Feather Stars, which are distinguished by their long and slender ‘arms,’ usually ten or more in number, each of which bears a number of pinnules that give it quite a feathered appearance. The second includes the Brittle Stars, possessing five slender arms that are jointed to the small, flattened, central disc, and which are so named on account of the readiness with which the animal falls to pieces when alarmed or disturbed; and the third is formed by the remaining five-rayed stars, the arms of which, instead of being jointed to, are continuous with, the centre of the body.
All these starfishes have a leathery skin, supported and hardened by a framework of calcareous plates, and presenting a number of hard ridges or spines. In addition to the system of water tubes already mentioned as characteristic of the echinoderms, they also possess a second circular vessel round the mouth, from which a number of vessels are distributed to the walls of the digestive tube. These, however, are bloodvessels, and are directly concerned with the nutrition of the body. Some, also, have imperfectly developed eyes at the ends of the arms or rays.
Contrary to what one would expect after watching the somewhat sluggish movements of starfishes, they are really very voracious creatures, attacking and devouring molluscs and small crustaceans, sometimes even protruding their stomachs to surround their prey when too large to be passed completely through the mouth; and they are also valuable as scavengers, since they greedily devour dead fishes and other decomposible animal matter.
Feather Stars differ from other starfishes in that they are stalked or rooted during one portion of their early life. At first they are little free-swimming creatures, feeding on foraminifers and other minute organisms that float about in the sea. Then they settle down and become rooted to the bottom, usually in deep water, at which stage they are like little stalked flowers, and closely resemble the fossil encrinites or stone lilies so common in some of our rock beds, and to which they are, indeed, very closely allied. After a period of this sedentary existence, during which they have to subsist on whatever food happens to come within their reach, they become free again, lose their stalks, and creep about by means of their arms to hunt for their prey.
Fig. 105.—Larva of the Feather Star
Fig. 106.—The Rosy Feather Star
The commonest British species of these starfishes is the Rosy Feather Star (Antedon rosaceus); and as this creature may be kept alive in an aquarium for some considerable time without much difficulty, it will repay one to secure a specimen for the observation of its habits. It is not often, however, that the Feather Star is to be found above low-water mark, its home being the rugged bottom under a considerable depth of water, where a number usually live in company; but there is no difficulty in obtaining this and many other species of interesting starfishes in fishing towns and villages where trawlers are stationed, for they are being continually found among the contents of the net.
Although the Feather Star can hardly be described as an active creature, yet it will cover a considerable amount of ground in the course of a day, creeping over rocks and weeds by means of its arms, which are raised, extended, and again depressed in succession, each one thus in turn serving the purpose of a foot. These arms are capable of being moved freely in any direction, as are also the little more or less rigid pinnules appended to them. The latter are bent backwards on an extended arm that is being used to pull the animal along, so that they form so many grappling hooks that hold on the bottom; and then the arm in question is bent into a curve by the contraction of its muscles, thus dragging the body forward. The arms on the opposite side of the body are also used to assist the movement by pushing it in the same direction, and this is accomplished by first bending the arms, and then, after curving the pinnules in a direction from the body, again extending them. Other movements of the feather star are equally interesting. Thus, the manner in which it will suddenly extend its arms and apply its pinnules to the surface on which it rests in order to obtain a good hold when alarmed, and the way in which it apparently resents interference when one of the arms is touched, are worthy of observation. The arms themselves are readily broken, and will continue to move for some time after being severed from the body, but the loss to the animal is only temporary, for a new arm grows in the place of each one that has been broken off.
This tendency to break into pieces is much greater in the Brittle Stars, as might be expected from their popular name; and is, in fact, such a marked characteristic of the group that it is not by any means an easy matter to obtain a collection of perfect specimens. They will often snap off all their arms, as if by their own power of will, when disturbed or alarmed, and even when removed from their hold without injury, they will frequently break themselves into pieces if dropped into spirit or in any way subjected to a sudden change of conditions.
The tube-feet of Brittle Stars are very small and are not provided with suckers, but are very sensitive, serving the purpose of feelers; also, having thin, permeable walls, they probably play a large part in the process of respiration. Both arms and disc are hardened by a dense scaffolding of calcareous plates; and not only are the former attached to the latter by a well-formed joint, but the arms themselves are constructed of a number of segments that are held together by a kind of ‘tongue and groove’ joint. Round the mouth are a number of tentacles that are kept in constant motion with the object of carrying the food towards it, and of holding the larger morsels while the act of swallowing is progressing.
Fig. 107.—The Common Brittle Star
The various species of Brittle Stars live among the rocks and weeds, chiefly in deep water, where they move about by means of the muscular contraction of their arms, the disc being raised on the curved arms as the animal proceeds. Some species are to be found between the tide-marks, and especially abundant on the south-west coast are two small species that live among the tufts of coralline weeds, sometimes so crowded together that dozens may be taken from a little patch of coralline only two or three inches square. These have such small discs, and such slender arms, and are, moreover, so well concealed by their colouring, which closely resembles that of the weed-tuft they inhabit, that they are only to be detected by close inspection.
The remaining division of the starfishes, sometimes distinguished by the name of Common Stars, possess five arms or rays, which may be either long or short, and which are not jointed with the central disc, but continuous with it; that is, there is no sharp line of demarcation between arm and disc. One or two species are well known to all frequenters of the sea-side, but the majority of them inhabit deep water, where they creep about over the rocks and weeds, obtaining their food from the bed below them.
If we examine the common five-finger star that is so often stranded on the beach, and so frequently found in rock pools between the tide-marks, we see that each arm has a large and conspicuous groove running along its centre on the under side, and on each side of these are the rows of tube-feet, arranged in such a manner that they have suggested the appearance of an avenue of trees on each side of a garden walk, and have consequently earned the name of ambulacrum. These tube-feet may be protruded for some distance; and, being provided with suckers that possess considerable clinging power, they form the principal means of locomotion.
Put the starfish in the aquarium, or in a tidepool by the sea, and you will find it very interesting to observe how the animal progresses, while some idea of the clinging power of the tube-feet may be ascertained by allowing the animal to creep over the submerged hand.
The movements of the tube-feet may also be seen to advantage when the starfish is laid upside down in a pool, and, what is still more interesting, the manner in which the animal turns itself over. To do this it will first bend the tips of one or two of its arms so as to bring the suckers against the ground; and then, aided by the pulling action of these, it will gradually bring other suckers into a similar position till, at last, the whole body has been turned over. Some of our common starfishes have rays so short that they may be termed angles rather than arms, and these are unable to turn their inverted bodies by the gradual method just described. They generally raise their bodies on the tips of three or four of the rays, assuming somewhat the form of a three-or four-legged stool, and then, bending the remaining one or two arms over the body, they alter the position of the centre of gravity till eventually the body topples over to the desired position.
Some of the common five-rayed stars have no suckers on their tube-feet, and consequently have to creep by means of the muscular contractions of their arms; and several of them are like the brittle stars in breaking up their bodies when irritated or seized. This latter peculiarity will account for the frequency with which we come across animals with one or more rays smaller than the others, the smaller rays being new ones that have been produced in the place of those lost. Again, we sometimes meet with such monstrosities as a five-rayed star with six or more rays, some smaller than others, the smaller ones representing two or more that have grown in the place of one that has been lost; or a starfish with branched or forked arm, illustrating the tendency to produce a new arm even when the original one has been only partially severed.
A close observation of a starfish in water may enable us to detect a number of little transparent processes standing out between the prominences of the rough skin of the upper surface. These are little bags filled with fluid, formed of such thin walls that gases can readily pass through them, and are undoubtedly connected with the process of respiration. Also, on the upturned extremity of each arm a red spot may be seen; and this from the nature of its structure, and from its association with the nervous system, has been regarded as a rudimentary eye.
On the upper side of the disc one may also observe a more or less conspicuous spot of variable colour, on one side of the centre. It is a plate, finely perforated, covering the outer extremity of a short canal which communicates with the system of water tubes that were described in the earlier part of this chapter. It is, in fact, the entrance through which water is admitted into the central ring round the mouth, and from this into the radial water tubes that run through each arm of the starfish to supply the tube-feet. The short tube referred to is always filled with sand, and thus the water that enters into the water-vascular system is filtered before it reaches the circular vessel. It is interesting to note, in this connection, that here is one respect in which the radiate symmetry of the starfish is broken, there being only one entrance, and that not a central one, by which water is distributed into the five rays.
Of course, when the ray of a starfish has been broken off the water vessel or vessels that it contained are destroyed, as is also the prolongation of the stomach, in the form of a long, blind tube, that extended into it. But no inconvenience attaches itself to this loss, for the starfish has the power of reproducing even its lost viscera, as well as any of the five rays of the body that may be broken off.
We must briefly refer to one other feature of the common star, viz. the possession of small prehensile organs around the mouth. These are little spines, the extremities of which are movable, and take the form of little pincers by means of which the animal can hold its prey.
When it is desired to preserve starfishes for future study, immersion in diluted spirit or a solution of formaldehyde will answer all purposes, the soft parts being thus preserved as well as the harder structures; but it is usual to preserve them in a dry state when they are required merely for purposes of identification, as is usually the case with the specimens in an ordinary museum collection. In the latter case it is advisable to put the starfishes in strong spirit for a few days, changing the spirit if several specimens are put together, and then drying them as quickly as possible in the open air.
Fig. 108.—Section of the Spine of a Sea Urchin
We have now to consider the Sea Urchins or Sea Eggs, which are readily known by the hedgehog-like covering of hard spines. Externally they appear as globular or heart-shaped bodies, the surface entirely hidden by spines except, perhaps, the mouth on the under side, which is provided with an apparatus for mastication. If alive, and in the water, one may notice that the animal creeps along the bottom, mouth downwards, moving itself either by means of its moveable spines, or by soft tube-feet resembling those of starfishes, that are protruded between the spines, or by both combined; and the movements of its masticating organ may be seen by observing the animal through the side or bottom of a glass vessel of sea water. The last-named organ is surrounded by an area of soft skin, and is not present in all species.
A closer examination of the common globular urchin will show that it is wonderfully constructed. Even the spines, which are in themselves uninteresting objects to the naked eye, are most beautifully formed, a transverse section revealing a radiate or reticulated structure when viewed through the microscope. Each spine has a concave base which fits on a little tubercle of the calcareous shell or test that covers the body of the animal, forming a perfect ball-and-socket joint, and is capable of being moved in any direction by means of small muscular bands.
Fig. 109.—Sea Urchin with Spines Removed on one side
Fig. 110.—Apex of Shell of Sea Urchin
On removing the spines the shell is seen to completely enclose the animal with the exception of the mouth, with its masticatory apparatus, and the small area around it which is covered by the uncalcified skin just referred to.
At the very top of the shell, exactly opposite the mouth, there is a small plate perforated by the extremity of the digestive tube. Round this are five angular plates, each perforated by the ducts of the ovaries or egg-producing glands, but one of these is enlarged and further perforated, that it may serve the second purpose of allowing water to enter the system of water tubes that supply the tube-feet, and thus corresponds exactly with the plate already noticed on the upper surface of the starfish. Between these are five smaller plates, each with a rudimentary eye that receives a fine nerve-thread.
The remaining and greater portion of the shell of the urchin is composed of ten radiating segments, each of which is made up of two rows of flat angular plates united at their edges. Five of these segments, arranged alternately with the others, are perforated by numerous holes, through which the tube-feet of the urchin are protruded, while the remainder are imperforate; and all ten plates bear the little hemispherical processes to which the spines are jointed.
Fig. 111.—Shell of Sea Urchin with Teeth protruding
One of the most interesting features of this urchin is undoubtedly its complex and wonderful masticating system. There are five teeth, symmetrically arranged, and all pointing towards the centre of the mouth. Each is attached to a wedge-shape jaw, made up of several pieces, and the whole apparatus is attached by ligaments to loops in the interior of the shell, and is moved by no less than thirty distinct muscles. The complete system may be readily dissected out, and is well worthy of study and preservation. (The harder portions of the system may often be found in the interior of the empty shell of an urchin after the softer structures of the body have decayed away.)
Fig. 112.—Interior of Shell or Sea Urchin
Fig. 113.—Masticatory Apparatus of Sea Urchin
An interesting dissection of the globular urchin may also be made by cutting completely round the shell with a pair of sharp-pointed scissors midway between the mouth and the apex, and then separating the upper and lower halves, as shown in fig. 114. In this way the whole of the digestive tube, with its numerous curves, may be traced from the mouth to the anus at the opposite pole. The water-vessels that supply the tube-feet in the regions of the five perforated plates may also be seen, as well as the ovaries or egg-producing organs and the bases of the five jaws with their complicated system of muscles.
A little acquaintance with the commonest of the British sea urchins will show that they may be divided into two well-defined groups, one containing the globular or subglobular forms, of which the common sea urchin or sea egg (Echinus sphæra) above described, is a type, as well as the pretty little Green Pea Urchin (Echinocyamus pusillus), and the little Purple-tipped Urchin (Echinus miliaris), which is found principally on the west coast of Scotland; while the second group is formed by the less symmetrical Heart Urchins, which differ from the others in several interesting particulars of structure and habit.
Fig. 114.—Sea Urchin Dissected, showing the Digestive Tube
These heart urchins (Plate IV., fig. 4) are covered with short, delicate spines which are not much used for purposes of locomotion, the animals moving from place to place almost entirely by means of their tube-feet, while the globular urchins travel principally by their spines, which are stouter and more freely moved on well-formed ball-and-socket joints. Also, while in the globular species the perforated plates that admit of the protrusion of the feet are arranged with a perfect radiate symmetry, those of the heart urchins are confined to one side of the shell; and the digestive tube, which in the former terminates in the pole opposite the mouth, in the latter ends close to the mouth itself. Further, the heart urchins do not possess any kind of dental apparatus.
Plate IV
ECHINODERMS
| 1. Asterias rubens | 4. Echinocardium cordatum |
| 2. Goniaster equestris | 5. Echinus miliaris |
| 3. Ophiothrix fragilis | 6. Echinus esculentus |
The habits of sea urchins are interesting, and may be watched in the aquarium, where the movements of the spines and of the tube-feet may be seen perfectly. Some species are very inactive, living in holes and crevices, or under stones, and seldom move from their hiding-places, while others travel considerable distances. The former have generally no eyes, and, instead of seeking their food, simply depend for their subsistence on the material carried to them by the movements of the water; while the latter possess visual organs similar to those observed in certain starfishes. Some species also protect themselves from their enemies when in the open by covering their bodies with sand, small stones, shells, or weeds, and thus so perfectly imitate their surroundings that they are not easily detected. The feet that are used for purposes of locomotion terminate in suckers resembling those of the common five-fingered starfish, and have considerable clinging power, but some have either very imperfectly developed suckers or none at all, and are probably used as feelers only.
Sea urchins, like their allies the starfishes, generally inhabit deep water beyond low-water mark, where they often exist in enormous numbers, feeding on both animal and vegetable substances; but some species are often to be met with between the tide-marks, where they may be seen under stones, and frequently half hidden in mud. The globular species occur principally on rocky coasts, but the heart urchins are more commonly dredged from banks of sand or mud that are always submerged.
The life-history of urchins closely resembles that of starfishes, for the young are free-swimming creatures of an easel-like form, and during this early larval existence their bodies are supported by a calcareous skeleton.
We will conclude our short account of the British echinoderms with a description of the peculiar Sea Cucumbers, which belong to the division Holothuroidea. These creatures are so unlike starfishes and urchins in general appearance that the uninitiated would hardly regard them as close relatives. The body is, as the popular name implies, cucumber-shaped, with the mouth at one end, and the general aspect is wormlike. There is, however, a radiate symmetry—a five-fold arrangement of parts, though not so regular as in most echinoderms. Running lengthwise along the body are five rows of tube-feet, but only two of these are well developed and terminate in functional suckers; and, as might be expected, the animal crawls with these two rows beneath it. The feet are outgrowths of a system of water tubes similar to that of the urchin, there being a circular tube round the mouth, from which branch five radial tubes, one for each row.
The mouth of the sea cucumber is surrounded by plumed tentacles which can be retracted at will, and which are used in capturing the smaller living things that form its food. Like the earthworm, it will often swallow large quantities of sand, from which it digests the organic matter contained.
The body-wall of the Holothuroidea is strong and muscular, and is strengthened by the presence of numerous spicules of carbonate of lime, often in the form of little anchors, wheels, and crosses, while the outer surface is rough and slimy, and often of a colour so closely resembling the surroundings of these animals that they are not easily observed. This feature is one of great value to the creatures, since they have no means of defence from their enemies, and seem to owe their safety entirely to their protective colouring.
Fig. 115.—The Sea Cucumber
There are several species of sea cucumbers on our coasts, but all inhabit deep water and are seldom to be seen above low-water level. They are, as a rule, easily obtained from fishermen, who will bring them in when requested to do so. Live specimens may be kept for a considerable time in the indoor aquarium, and seem to prefer a rocky bottom on which they can hide under stones at times, and a bed of sand on which they will occasionally crawl. They will readily devour small molluscs and crustaceans, and will partake of dead organic matter in a partially decomposed state.
The following tabular summary of the classification of Echinoderms may possibly be of use for reference:—
SUB-KINGDOM ECHINODERMATA
| Body star-shaped | Body globular, subglobular, or heart-shaped, and covered with a continuous shell. Class: Echinoidea (Sea Urchins). | Body elongated, and covered with a soft skin containing calcareous spicules. Class: Holothuroidea (Sea Cucumbers). | ||
Body stalked, at least during early stage. Feet not used for locomotion. Class: Crinoidea (Feather Star). | Body not stalked. Tube-feet used for locomotion. Class: Stelleridæ. | |||
Arms jointed to disc, and not containing prolongations of the internal organs. Order: Ophiuroidea (Brittle Stars). | Arms continuous with disc, and containing processes of the viscera. Order: Asteroidea (Common Stars). | |||
CHAPTER XI
MARINE WORMS
Some groups of animals are so well defined that the individual species contained in them can be assigned their proper place without any difficulty, the main characteristics by which the group is distinguished running with more or less precision throughout the series; but, unfortunately this is not the case with the ‘worms,’ which constitute the sub-kingdom Vermes. Here we have a most heterogeneous assemblage of animals, collectively exhibiting exceedingly wide variations in both form and structure.
We have already referred to the sea cucumber as wormlike in form, and this creature is only one of a large number of wormlike animals that are not worms; and it is also a fact that a considerable number of the worms are not wormlike. It appears as if the sub-kingdom Vermes were a kind of receptacle into which we may throw almost any invertebrate animal that does not readily fall in line with the general characteristics of the other important groups; for in it we have such a varied assemblage of creatures that, speaking of them collectively as worms, it becomes most difficult, if not absolutely impossible, to say exactly what a worm is; and it is a question whether the sub-kingdom ought not to be divided into at least two or three groups of the same standing.
This being the case we can hardly give a satisfactory summary of the characteristics of the group, and therefore it must be understood that in our attempt to do so we unavoidably exclude some forms that belong to it according to our present system of classification. This being remembered, we will define worms as soft-bodied and elongated animals, exhibiting a bilateral symmetry (that is, having appendages and organs arranged symmetrically on each side of a plane extending from the dorsal to the ventral surface through the centre of the body), and with the body usually divided into a succession of segments, each of which resembles the one preceding and following it. Though many of the worms are generally looked upon as uninteresting creatures, of such an unattractive appearance and with such depraved habits that they are beneath respect, yet a study of the sub-kingdom will prove that not only does it include a number of wonderful forms with the most marvellous life histories, but that some of them are very beautiful objects; and this last remark refers more particularly to many of the marine worms, which come directly within the scope of our work.
Before passing on to the special study and classification of the marine species, however, we must say a few words concerning the worms in general, reminding the reader that all our statements regarding the anatomy of the creatures may be readily verified by simple dissections of one or two typical species, such as the common earthworm, the fisherman’s lugworm, the sea mouse, or the common horse-leech of our fresh-water ponds. With this object in view, the animal may be killed by immersion in spirit, then pinned out in the dissecting tray under water, and the body-wall opened by means of a pair of sharp-pointed scissors.
The digestive tube of a worm runs completely through the length of the body, and though there is no distinct head, there is always a mouth, and this is often provided with horny jaws, and sometimes also with horny teeth, with which the animal is enabled to inflict wounds on its prey.
Like the preceding sub-kingdom—the Echinodermata—worms possess a system of water tubes; this system, however, is not in any way connected with the function of locomotion, but is, in many cases at least, if not in all, intimately associated with the process of respiration. It consists of a series of tubes, arranged in pairs in the successive segments, communicating with the body-cavity internally, and opening at the exterior by means of pores in the cuticle. In some there is a highly organised system of bloodvessels, containing blood that is usually either colourless, red, or green, but the colour of the blood is never due to the presence of corpuscles, as is the case with higher animals, the tint being due to the plasma or fluid portion of the blood; and though worms cannot be said to possess a true heart, yet they often have one or more contractile bloodvessels which serve the purpose of propelling the blood.
Most worms possess a nervous system, and, where this is present, it consists of a chain of ganglia, placed along the ventral side of the body, beneath the digestive tube, all united by means of a nerve cord, and distributing nerves in pairs to various parts of the body; and it may be well to note here one very important point of distinction between the general arrangement of the central portion of the nervous system in the worms and higher invertebrates, as compared with that of the corresponding structure in the vertebrates:—In the former the main axis of the system, consisting, as we have seen, of a chain of ganglia connected by a nerve cord, is invariably placed along the ventral portion of the body-cavity—the surface on which the animal crawls; while in the vertebrates the axis of the nervous system lies along the upper or dorsal part of the body; and, instead of lying in the general body-cavity, in company with the organs of digestion and circulation, is enclosed in the bony canal formed by the vertebral column. It will be seen from this that when it is desired to examine the nervous system of the invertebrate animal, the body-wall should be opened along the middle of the ventral surface, while, in the vertebrate, the central axis should be exposed from above.
Many of the vermes are parasitic, either attaching themselves to the exterior of other animals, and deriving nourishment by sucking their blood, or they are internal parasites, living in the digestive canal of their hosts and partaking of the digested food with which they are almost perpetually surrounded, or burrowing into the tissues and imbibing the nutritive fluids which they contain; and it is interesting to study even these degraded members of the group, if only to observe how their physical organisation degenerates in accordance with their depraved mode of living. In them we find no digestive system with the exception of the simplest sac from which the fluids they swallow may be absorbed, for their food is taken in a condition ready for direct assimilation; and the food so obtained being readily absorbed into all parts of their soft bodies, and being sufficiently charged with oxygen gas by the respiration of their hosts, they require no special organs for circulation or respiration, nor, indeed, do we find any. Further, we find that the nervous system is often undeveloped; for since the parasites, and especially the internal ones, are so plentifully surrounded with all the necessaries of existence, their bodies are so simple in construction that no complex nervous system is required to promote or control either locomotion or internal functions. Even the general body-cavity often disappears in these degraded creatures, the internal organisation being of such a low type that there is no necessity for it; and all the abundant nourishment absorbed over and above that required for the sustenance of their simple bodies is utilised in the reproduction of the species; consequently we find, as a rule, the reproductive organs well represented, and the species concerned very prolific.
It is an interesting fact, too, that these parasites, in their earliest stage, possess organs which are present in the higher worms, but which degenerate as they approach the adult form, thus indicating that they have descended from more respectable members of the animal world, and that the low physical development which they ultimately attain is the natural result of their base mode of living.
The young marine naturalist, working on our coasts, will not be brought into intimate contact with parasitic worms to any large extent, yet we have said this little on parasitism to show that these degenerate creatures are not really devoid of interest, and that they will repay study whenever they are found. They will be more frequently met with during the examination of the animals—usually higher types—that become their hosts, and thus they hardly come within the scope of this work.
Fig. 116.—A Turbellarian, magnified
a, mouth; b, cavity of mouth; c, gullet; d, stomach; e, branches of stomach; f, nerve ganglion; g to m, reproductive organs.
The simplest of the worms are those forming the class Turbellaria, so designated on account of the commotion they produce in the water surrounding them by means of the vibratile cilia that fringe their bodies—a characteristic that is also expressed by their popular name of Whirl Worms. They are usually small creatures, with soft, flattened, unsegmented bodies, though some of the larger species are really wormlike in form, and are more or less distinctly divided into a chain of segments. Many of them are marine, and may be seen gliding over stones left uncovered by the receding tide with a smooth slug-like motion, and when disturbed in a rock pool, occasionally swimming with a similar smooth motion by the aid of their cilia. They avoid bright light, and are consequently generally found on the under surfaces of stones, especially in rather muddy situations, and where the stones are covered with a slimy deposit of low forms of life. In these turbellarians the mouth is situated on the under surface, thus enabling the animal to obtain its nourishment from the slimy surface over which it moves, and it is also provided with an extensile proboscis that aids it in the collection of its food. The digestive tube is generally very complex in form, extending its branches into every part of the soft body; and, there being no special organs of respiration, the animal derives all the oxygen required by direct absorption from the water through the soft integument.
When searching for turbellarians on the sea shore one must be prepared to meet with interesting examples of protective colouring that will render a close examination of rocks and stones absolutely necessary. Some of these worms are of a dull greyish or brownish colour, so closely resembling that of the surface over which they glide that they are not easily distinguished; and the thin bodies of others are so transparent that the colour of the stone beneath is visible through them, thus preventing them from being clearly observed.
Overturned stones should be examined for their flattened bodies gliding along rapidly in close contact with the surface. They may be removed without injury by placing a wet frond of a sea weed close to the stone, in front of one end of the body, and then urging them to glide on to it by gently touching the opposite end. Sometimes, however, the turbellarians remain perfectly still when exposed to the light, in which case they are even more difficult to detect, but a little practice will soon enable one to distinguish them with readiness.
Allied to the turbellarians are the Spoon Worms or Squirt Worms, some species of which inhabit deep water round our shores, where they burrow into the sand or mud of the bed of the sea. These form the class Gephyrea, and consist of creatures with sac-like or cylindrical and elongated bodies, and a protrusible proboscis, which is often of great length. Their bodies are not distinctly segmented, nor do they bear any appendages. The skin is tough and horny, and the body-wall, which is very thick and muscular, is often contracted when the animal is disturbed, thus causing a jet of water to be forcibly ejected.
All the most interesting of the marine worms belong to the Annelida or Chætopoda, popularly known as the Bristle-footed worms, because their locomotion is aided more or less by the presence of stiff bristles that project beyond the surface of the skin. These are all highly organised worms, mostly with very elongated bodies that are distinctly segmented exteriorly by a number of transverse grooves, while the interior is correspondingly divided into a number of compartments by means of a series of septa.
In addition to the bristles already mentioned, there are often numerous appendages, but these must be distinguished from the more perfect appendages of the arthropods, to be hereafter described; for while the latter are distinctly jointed to the body, and are themselves made up of parts that are jointed together, the former are mere outgrowths of the body-wall. The digestive and circulatory systems are well developed, as is also the system of water tubes that connect the body-cavity with the exterior, while the body-cavity itself is full of fluid.
This group of worms is subdivided into two divisions, the many bristled (Polychæta) and the sparsely bristled (Oligochæta) worms. The latter contain the common earthworms and some less known species, while the former include a number of interesting and even beautiful worms, all of which are marine, and many of them among the commonest objects of the sea shore.
These Polychætes exhibit a great variety of habit as well as of appearance. Some live in crevices of the rocks or under stones and weeds, or make burrows in the sand or mud of the bed of the sea, and roam about freely at times in search of food. They are continually coming within the ken of the sea-side collector, being revealed by almost every overturned stone near the low-water mark, and are often seen crawling over the wet rocks just left uncovered by the receding tide; while their burrows are often so numerous that hundreds may be counted in every few square feet. But many are sedentary species, and these are not so generally known to young sea-side naturalists, who frequently observe, and even preserve, the interesting homes they construct, while less attention is given to the architects that build them.
It is very interesting to observe some of the general differences between the roving and the sedentary species—differences which illustrate the principle of adaptation of structure to habit. The roving species are provided with a lobe that overhangs the mouth, bearing feelers and eyes, and are thus enabled to seek out any desired path and to search for their food. They are provided with bristles and other appendages by means of which they can travel freely over the surfaces of solid objects, and are able to swim well either by undulations of the body, or by fringed appendages, or both. The carnivorous species, too, are provided with strong, horny jaws, and sharp, curved teeth, by means of which they can capture and hold their prey. The sedentary species, on the other hand, unable to move about in search of food, are supplied with a number of appendages by means of which they can set up water currents towards their mouths, and which also serve the purpose of special breathing organs, and, having no means of pursuing and devouring animals of any size, they do not possess the horny jaws and curved teeth so common in the rovers. Their eyes, too, are less perfectly developed, and the tactile proboscis of their free-moving relatives is absent.
Fig. 117.—Arenicola piscatorum
Of the roving worms, perhaps, the Lugworm or Sandworm (Arenicola piscatorum) is the best known. Its burrows may be seen on almost every low sandy or muddy shore, and, being so highly valued as a bait, its general appearance is well known to all professional and amateur sea fishers. It reaches a length of eight inches or more, and varies in colour according to the sand or mud in which it lives. The segments of this worm are very different in structure in different parts of the body. Those in the front of the body have a few tufts of bristles arranged in pairs, while the middle portion of the body has large brush-like tufts of filamentous gills placed rather close together; and the hindmost part has no bristles or appendages of any kind, and is so well filled with the sand or mud that it is quite hard and firm to the touch. As is the case with our common earthworms, the sand or mud is swallowed in enormous quantities, and this is not only the means by which the lugworm derives its food, but also assists it considerably in making its burrows; the extent to which this creature carries on its work of excavation may be estimated by the thousands of little contorted, worm-like heaps of sand that lie on the surface at every period of low water. These little heaps are known as ‘castings,’ and consist of the sand that passed through the worm’s body as the burrowing proceeded.
The Ragworm is another species that is highly valued as bait. It burrows into the odorous mud that is so commonly deposited in harbours and the mouths of sluggish rivers. In this species the segments are similar throughout the length of the body, and the numerous flattened appendages give it the ragged appearance that has suggested its popular name. Quite a number of marine worms closely allied to the common ragworm, and resembling it in general form, are to be found on our shores. Many of these may be seen by turning over stones that are left exposed at low tide, while others hide themselves in snug little crevices of the rock, or in the empty shells of the acorn barnacle and various molluscs; and some species, including one of a bright-green colour, creep freely over the wet rocks in search of food or home, often exposing themselves to the rays of a fierce summer sun.
Fig. 118.—The Sea Mouse
The Sea Mouse (Aphrodita aculeata) is certainly one of the most interesting of the roving marine worms, and, though seldom seen above low-water line, may often be obtained by the sea-side collector with the aid of friendly fishermen, who sometimes find it plentifully among the contents of their trawl nets. Failing such aid, it may be looked for among the encrusted stones that are exposed only at the lowest spring tides, especially in places where a certain amount of mud has been deposited under the shelter of outlying rocks; and the chances of success are much greater if the search is made immediately after a storm, for at such times much of the life that exists in deep water will have been driven shoreward by the force of the waves.
At first sight the sea mouse would hardly be associated with the worms; for, instead of having the elongated and cylindrical form that is usually regarded as characteristic of these creatures, it is broad and slug-like in shape, the under surface, on which it crawls, being flat, while the upper side is convex. The segmentation of the body, too, is not readily seen in the upper surface on account of the thick felt-like covering of hairs, but is at once apparent when the creature has been turned over to expose the ventral side.
When seen for the first time in its natural haunt one naturally wonders what the moving mass may be. Crawling sluggishly over incrusted stones, or remaining perfectly still in a muddy puddle that has been exposed by overturning a stone, it looks like a little mound of mud itself, about four or five inches long, and its general colour and surface so closely resembles that of its surroundings that an inexperienced collector may never even suspect that the mass is a living animal form. But take the creature and wash it in the nearest rock pool, and it will be recognised as a broad segmental worm, thickly covered with fine hairs above, and its sides adorned by bristles that display a most beautiful iridescence. It is not easy to see the value of this gorgeous colouring to the animal, and it is doubtful whether, on account of the muddy nature of the creature’s home, such colouring is often displayed to the view of other inhabitants of the sea; but it is well known, on the other hand, that sea mice are readily devoured by fishes, even though they possess an armature of stiff and sharp spines, and that they must therefore be often preserved from destruction by the close resemblance of the general colour to that of their surroundings.
The gills of the sea mouse are not prominent appendages, as with most marine worms, but are soft fleshy structures situated beneath the overlapping scales that lie hidden below the thick hair of the upper surface.
As it is most probable that the reader may desire to preserve a sea mouse at some time or other, a few words concerning the best methods of doing this may be of value. If it is to be preserved in fluid, it should be thoroughly washed to remove all the mud that normally covers its body, and then placed in spirit or formaldehyde, both of which fluids have no destructive effects on the iridescent colouring of the bristles. If, however, it is desired to keep the specimen in a dry state, it should first be put into strong spirit containing a few grains of corrosive sublimate, for a few days. It should then be put under considerable pressure between several thicknesses of absorbent paper to expel the fluid it contains, as well as all the softer internal structures. By this means it will have been squeezed quite flat, so that it presents anything but a natural appearance; but the skin may be blown out to the normal shape by means of a glass tube inserted into the mouth, and then set aside to dry. As the water it originally contained has been extracted by the strong spirit, the drying takes place very quickly; and the small amount of corrosive sublimate that has penetrated into its substance will be sufficient to protect it from the invasion of those pests that commonly attack our museum specimens.
Passing now to the sedentary or fixed worms, we meet with some that are very interesting and beautiful creatures, even when considered apart from the wonderful homes they construct. The several species of the genus Terebella form a soft and flexible tube by binding together particles of sand, shells, or mud with a sticky substance that exudes from their own bodies. These tubes are to be found in abundance between the tide-marks on almost every low, sandy shore, the nature of the tubes varying, of course, with the character of the materials at the disposal of the builder.
In some cases the tubes are exposed throughout the greater part of their length, but very frequently they are more or less buried in the sand or other material of the beach, so that one has to dig to a moderate depth in order to extricate them. In either case, however, the tube of Terebella may be known by the free tufts of sandy threads that form a deep fringe around its mouth.
These worms almost invariably select a sheltered situation for their abode, and should be searched for at the foot of rocks, or under stones, and it is no easy matter to move the buried tube with its occupant intact.
When turning over the stones of a sandy or muddy beach one frequently discovers the slender, thread-like tentacles of the Terebella, together with the sandy filaments that surround the mouth of the tube, the remainder of the tube and its occupant being beneath the surface, and the ground is often so hard and stony that a strong tool is necessary to dig it out; but the work entailed will be amply repaid if a perfect specimen be obtained and placed for observation in the aquarium.
Fig. 119.—Tube-building Worms: Terebella (left), Serpula (middle), Sabella (right)
The reader may possibly be acquainted with the tubes or cases that are constructed by the larvæ of caddis flies in fresh-water ponds and streams, and perhaps has noticed the ease with which these creatures may be made to construct new homes after having been turned out of doors. Similar experiments may be performed with Terebella; for when the worm has been extricated from its tube without injury—a work that requires great care on account of the soft and slender nature of the creature’s body—and placed in the aquarium with a bed of suitable material, it will build itself a new dwelling. As with the caddis larvæ, the different species may be known by the materials they select to construct their tubes, but in captivity they may be compelled to employ other than their favourite substance for this purpose. It is unfortunate, however, that Terebella is a nocturnal builder, and thus its movements are not so easily observed.
When removed from its tube its first movements suggest a resentment at the untimely ejection. This being over, it seeks a sheltered situation beneath the edge of a stone, and, at nightfall, commences the slow process of the construction of a fresh home. The particles of material at hand are seized by the tentacles, placed in position round the body, where they are held together by the sticky secretion already mentioned.
Fig. 120.—Terebella removed from its tube
The tentacles are employed in two distinct ways:—They may be flattened into slender ribbon-like structures, which, by being folded longitudinally at any point, may be made to grasp a particle of sand; and, in addition to this, the tip of the tentacle may be converted into a minute cup-shaped sucker by the withdrawal of the fluid it contains into the body.
Some species of Terebella build their tubes of ordinary sand, while others select fragments of shells. Some employ mud only, and occasionally we meet with tubes constructed of the silky secretion of the body with hardly any foreign matter.
We sometimes see edges of rocks, on low, sandy shores, covered with what appears to be large masses of consolidated sand, full of holes a little more than an eighth of an inch in diameter; and these masses are often so extensive and so firm that they seem to form the greater part of the rock itself. Such masses are particularly abundant on the south coasts of Devon and Cornwall, but are more or less plentiful on most sandy shores of Great Britain. They consist of the tubes of a species of the marine worm Sabella, which have been built up much in the same manner as those of Terebella, but usually exist in such numbers in the same spot that, together with the sand that has been washed between them, they form the dense masses just described.
A cluster of some dozens of these tubes may be detached with the aid of a hammer and chisel; or, in some instances, where the mass of tubes is not held so firmly together, by the mere pressure of the hand; and it will then be observed that each tube consists of a flexible membrane, of a somewhat leathery nature, formed by a sticky secretion from the body of the worm, with its outer surface covered with grains of sand. The tubes may be easily opened, and the occupants extracted for examination, when it will be observed that the front or upper portion of the worm is short and thick, while the hindmost portion is much thinner, and is doubled forwards in the tube. The body is also provided with numerous bristles, by means of which the worm is enabled to grasp the membranous lining of the tube, and thus secure a firm hold within its home.
A cluster of these tubes should be placed in a rock pool, or in the marine aquarium, when the worms may be seen to protrude gradually, and expose a large number of feathered tentacles, which, by their incessant motion, keep up the constant circulation of the water for the purpose of respiration as well as to bring food particles towards the mouths of the worms.
It is possible to keep these worms alive for some time in the aquarium, but special care is required for the reason that it is a very difficult matter to secure a cluster of tubes without injury to a certain number which are sure to be broken or otherwise damaged; and these, dying and decomposing within their homes, speedily pollute the water. Hence it is necessary to keep a sharp watch for dead specimens, which should, of course, be removed at once. The presence of a putrefying worm may often be detected by the appearance of a whitish fungoid growth round the mouth of what appears to be an empty tube; and if, through neglect, the water of the aquarium has been allowed to become contaminated by the products of decomposition, it will often happen that some of the living worms will come entirely out from their tubes, as if to seek a more sanitary situation. Thus, the exit of worms from their homes may always be looked upon as pointing to a suspicious condition of the water which, if not corrected immediately, may lead to the death of all.
The species we have briefly described is by far the commonest of the genus Sabella, but there are several others to be found on our shores. Some are of a solitary nature, and construct a sandy tube so much like that of a certain species of Terebella that they may be mistaken for that genus. Another solitary species builds a hard stony tube of carbonate of lime that has been extracted from the sea water; and although it is hardly possible to take the live worm from this calcareous tube without injury, the animal may be obtained intact for examination or preservation by dissolving away the tube in dilute hydrochloric acid.
Fig. 121.—A tube of Serpula attached to a Shell
While engaged in collecting specimens on the sea shore we are continually meeting with stones and shells that are more or less covered with white, limy tubes twisted into all manner of serpentine forms. These are the tubes of other marine worms known as the Serpulæ, which, like the species previously mentioned, are interesting objects for the aquarium.
The tubes themselves are worthy of study and preservation, more especially as they vary in form, and may, to some extent, provide a means by which the different species may be identified. They are composed of fine layers of calcareous matter secreted by the body of the worm within, and lined by a thin leathery membrane which may be easily exposed by dissolving away the mineral matter as just described. Some are triangular in section, and often distinctly keeled, while others are cylindrical, and flattened more or less on the lower side. The triangular tubes are attached to stones or shells throughout their length, but the cylindrical ones are often elevated above the surface in the wider and newer part.
If a cluster of these tubes, freshly gathered from between the tide-marks, be placed in the aquarium, the worms will soon protrude the foremost portion of their bodies, exposing beautiful fan-like gills, often brilliantly coloured in shades of scarlet, blue, or purple, which are kept in motion in such a manner as to convey water, and consequently also food, towards the mouth. The gills are of course, richly supplied with blood, for their main function is to aërate that liquid by exposing it to the water in order to absorb oxygen gas. The body of the worm is provided also with little cilia, which, by their constant vibratory motion, keep up a circulation of water through the tube; and this not only keeps the tubular home free from excrement and other sedimentary matter, but also probably assists in the function of respiration by bringing fresh supplies of water in contact with the animal’s soft and absorbent skin.
Fig. 122.—Serpula removed from its Tube
When the worms are disturbed they immediately withdraw themselves within the tubes, this being done by the aid of the numerous minute hooklets on the surface of the body that enable the worms to cling firmly to the membranous linings of their homes; and it will then be observed that the mouth of each tube is closed by a lid (operculum), which hangs as by a hinge when not in use. These operculi vary much in character, and supply another aid in the identification of the various species. They differ much in shape, and may be either membranous, horny, or calcareous.
Little calcareous tubes, somewhat similar to those of the Serpulæ, but always in the form of a spiral, may often be seen on stones and shells, and the fronds of sea weeds, sometimes so closely packed together as to almost entirely cover the surface. The average diameter of these spirals is only about a sixteenth of an inch, and many are so small that a lens is necessary to discern their shape. In general form they closely resemble some of the small species of Planorbis shells that are so common in our ponds and streams, but these latter are the shells of freely moving molluscs, and are generally of a brownish colour.
Fig. 123.—The Sea Mat (Flustra)
The minute worms that live within the tubes in question belong to the genus Spirorbis, and are very similar to those of the Serpulæ, and their pretty plumed gills may be seen with a lens when a cluster of the tubes is placed in a shallow vessel of sea water. A sharp tap on the table on which the vessel rests will cause the little creatures to suddenly retire into their homes, the entrances to which may then be seen to be closed by an operculum.
There is an interesting group of animals known collectively as the Bryozoa or Polyzoa, or, popularly, as the Moss Polyps, that are often classed with the worms, though they are not, according to the general idea, wormlike in appearance. They live in pretty colonies, many of which are certainly familiar objects to all who ramble along the sea shore. Some form pretty lacelike patches on the fronds of sea weeds, while others are built up into flat, frond-like, branching objects that are often mistaken for sea weeds by young collectors. Among the latter is the Sea Mat (Flustra), that is so commonly washed up on the shore in great abundance. An examination with a lens will show that, in both instances, the mass consists of very many minute cells, with horny or calcareous walls, the mouth of each cell being close by an operculum.
On placing the colony in sea water, however, we find that each little cell is the home of a small animal, that protrudes from the cell, exposing a mouth that is surrounded by a crown of tentacles. A moderately high magnifying power will also show that the tentacles are covered with minute vibratile cilia, by means of which currents of water are set in motion towards the mouth to supply the animal with food. Some, too, have a lip by means of which the mouth may be closed.
Fig. 124.—Flustra in its Cell, magnified
In addition to the colonies just briefly described, there are other moss polyps that build up little, branching, tree-like clusters which closely resemble some of the sea firs, and many of these are to be found in the sheltered crevices of rocks, or attached to the under sides of stones between the tide-marks.
While searching the surfaces of rocks and weeds at low tide, one’s fingers will be constantly coming in contact with fixed, soft-bodied animals that suddenly eject a fine stream of water as they are touched. These are the Sea Squirts, sometimes spoken of as the Tunicate Worms. They are semi-transparent creatures of oval or elongated form, and usually of a pale yellow, brown, or pink colour; and derive their popular name from the fact that they are covered externally by a continuous tunic or wall of tough structure.
Although the tunicates resemble worms in many points of structure, it is interesting to note that in their young or larval state the body consists of two cavities, one of which contains the internal organs, while in the other the central portion of the nervous system is developed, in which respects they resemble the vertebrate or back-boned animals—fishes, amphibians, reptiles, birds, and mammals. At this stage, too, the creatures possess a tail that is supported by a rod of gristle similar to that which gives place to the backbone in the developing vertebrate. These features, though only transitory, are regarded as a mark of relationship to the higher forms of animal life, and thus the tunicates have been separated from the sub-kingdom Vermes by some zoologists, and given an exalted place at the top of the invertebrate scale, where they form a sub-kingdom of their own, and are looked upon as a link connecting the invertebrates with the vertebrates.
Fig. 125.—Sea Squirt
Before passing on to the next sub-kingdom, we should observe that the interesting Rotifers or Wheel Animals also belong to the Vermes; but although many of these minute creatures are to be found in sea water, their principal home is the stagnant water of fresh-water ponds and ditches, and thus we may be excused for neglecting them here.
CHAPTER XII
MARINE MOLLUSCS
The sub-kingdom Mollusca includes a great variety of soft-bodied animals which differ from the members of the last division in the fact that they are never segmented, and in the possession of a thick outer covering, of a leathery nature, which completely envelops the body, and which usually secretes a calcareous shell of one or more parts. A general idea of the extent of the group may be formed when we state that it contains the Octopus and the Cuttlefish; all Snails and Slugs, and animals of a similar nature; and all those numerous ‘bivalves’ which are represented by the well-known Oysters, Mussels, Scallops, &c.
By far the greater number of the molluscs are aquatic in habit; and of these such a large proportion are marine that the group provides plenty of occupation for the sea-side naturalist. This being the case, we shall devote the present chapter to a description of the general characteristics of these animals, and to the principles of their classification, illustrating our remarks by a few selections from all the chief divisions.
Although, as we have already hinted, the body of a mollusc generally bears but little resemblance to that of the typical elongated and segmented worm, yet the study of the earliest stages of the former shows that a certain relationship exists between the two sub-kingdoms, the newly hatched mollusc being often a minute free-swimming creature with expanded lobes fringed with cilia, and bearing a resemblance to certain of the Rotifers, Moss Polyps, and other animals that are included among the Vermes. But in the adult molluscs this resemblance is lost, these creatures being generally easily distinguished from all others by certain well-marked external features, as well as by internal characters that are peculiar to them and fairly constant throughout the group.
The external shell, where it exists, is usually composed of one or of two parts, and therefore we speak of univalve and bivalve molluscs; and no internal skeleton of any kind is to be found except in the division containing the Cuttlefishes, the ‘bone’ of which is one of the common objects washed up on our shores by the breakers.
In all the molluscs there is a well-formed digestive tube, and often a complex arrangement of small teeth which sever the food by a rasp-like action. There is also a well-formed heart, consisting of two or more cavities, by means of which the blood is forced through the body; but, as a rule, blood vessels are either few or absent, the blood being driven through spaces between the tissues that serve the same purpose.
Fig. 126.—Larvæ of Molluscs
v, ciliated ‘velum’; f, rudimental foot
The nervous system consists of a few masses of nerve substance (ganglia), connected by nerve cords, and sending off fibres to various parts of the body, the principal ganglion being one situated close to the mouth, and often surrounding the first portion of the digestive tube.
The animals of this sub-kingdom are grouped into three principal and well-marked divisions—the Lamellibranchs, or Plate-gilled molluscs, the gills of which are composed of plate-like layers, and the headless bodies enclosed in a bivalve shell; the Cephalophora, or head-bearing molluscs, protected by a univalve shell; and the Cephalopoda, or Head-footed molluscs, so called because the mouth is surrounded by tentacles or arms by which the animal can cling to objects or seize its prey.
We shall deal with these three divisions in the above order, taking first the bivalves, the shells of which are found in great variety along our shores.
The general nature of a lamellibranch is easily made out by the examination of one of the common species that may be obtained alive on any part of the coast, such as the Edible Mussel, the Cockle, or the Oyster, and the reader will do well to secure a few specimens and examine them with the aid of the following description of the principal distinguishing features.
The shell is formed of two valves, united by a hinge which is sometimes of the simplest possible description, but which often exhibits a beautiful arrangement of interlocking teeth. A ligament of flexible and elastic substance often holds the two valves together.
Fig. 127.—Shell of the Prickly Cockle (Cardium aculeatum) showing Umbo and Hinge; also the interior showing the Teeth
The reader has probably observed that the valves of a dead lamellibranch usually gape. This is due either to the pull exerted by a ligament that is attached to the valves outside the hinge, or to the pressure of an internal cartilage which unites the valves within, and which is compressed when the shell is closed. When the animal is alive, it has the power of closing its shell by the contraction of the adductor muscles, to be presently described, and when the valves are brought together by this means the external ligament is more or less stretched, or the cartilage within, which is also an elastic material, is compressed.
Examining the shell from the exterior we observe that each valve has a nucleus (the umbo) close to the hinge, round which are usually a number of more or less distinct concentric lines, extending to the lower or ventral margin. This nucleus represents the whole shell of the young mollusc, and the lines are the lines of growth, each one marking the extreme limit of the valve at a particular period of the animal’s existence. Further it will be observed that the lines of growth are often wider apart in some directions than in others, thus denoting the unequal rate of growth that determined the form of the adult shell.
Fig. 128.—Interior of Bivalve Shell, showing Muscular Scars and Pallial Line
The shell of a bivalve is often made up of two very distinct layers, the outer one called the prismatic layer because, when examined microscopically, it is seen to consist of minute vertical prisms of calcareous matter; and the inner one presenting a beautiful pearly iridescence, due to the fact that it is made up of a number of extremely thin and finely waved layers of calcareous substance that have the power of decomposing light. This latter layer is secreted by the whole surface of the mantle that lies in contact with it, while the outer, prismatic portion of the shell is formed only by the free edge of the mantle; and we often find a distinct line (the pallial line), some little distance from the ventral margin that marks the junction of the muscle of the mantle with the shell. The shape of this line is a very important feature of the shell, since it is of great value in the determination of relationships.
Further, the inner surface of each valve is marked by the impressions or scars of other muscles, the number and position of which vary considerably in different species. They include the adductor muscle or muscles (one or two in number) that pull the valve together; the muscle or muscles that withdraw the foot, called the retractor pedis, and the protractor pedis that pulls the foot out. Not only are these scars often very distinct in themselves, but we may frequently observe lines running tangentially from their circumferences towards the umbo, to which they all converge. These lines enclose the areas previously occupied by the muscular impressions; in other words, they show the directions in which the muscles named above shifted their positions as the animal grew.
Fig. 129.—Diagram of the Anatomy of a Lamellibranch
f, mouth, with labial palps; g, stomach; i, intestine, surrounded by the liver; a, anus; r, posterior adductor muscle; e, anterior adductor muscle; c, heart; d, nerve ganglion; m, mantle (the right lobe has been removed); s, siphons; h, gills; ft, foot
Now let us obtain a few species of live lamellibranchs, put them in a vessel of sea water, and observe them after they have been left undisturbed for a time. The shell will be seen to gape slightly, exposing the edges of the two lobes of the mantle which lie closely on the inner surface of the valves, thus completely enveloping the body of the animal; and at one end, usually the narrower end in the case of irregular shells, we shall observe two openings—the siphons, sometimes enclosed within a tube formed by a prolongation of the united mantle lobes, and protruding from between the valves, and sometimes formed by the mere contact of the mantle lobes at two adjacent points. If now we introduce a little carmine or other colouring matter by means of a glass tube, setting it free near the lower siphon—the one more remote from the umbo of the shell, we observe that it enters the body of the mollusc through this opening, and reappears shortly afterwards through the upper or dorsal siphon. Thus we see that water currents are incessantly circulating in the body of the animal, entering by the inhalent or ventral siphon, and leaving by the exhalent or dorsal siphon. These currents are maintained by the vibratile action of thousands of minute cilia belonging to cells that line the cavities of the body, and serve to supply the animal with both air and food; for lamellibranchs, being gill-breathers, derive the oxygen necessary for respiration from the air held in solution by the water, and their food consists entirely of the minute living creatures that always abound in natural waters.
Again, we shall find that some of our live bivalves have protruded a thick, conical, fleshy mass—the foot, from the opposite end of the body. This organ is the means of locomotion in the case of the burrowing and other free-moving bivalves, but is developed to a less extent in those species that lead a sedentary life. Thus, the common Edible Mussel secretes a tuft of strong silky fibres (byssus) by means of which it fixes itself to a rock or other body, and therefore does not need the assistance of a muscular foot; and an examination of its body will show that the foot is very small in proportion to the size of the animal, as compared with that of the wandering and burrowing species. The same is true of the oyster, which lies fixed on its side, the lower valve being attached to the surface on which it rests.
Fig. 130.—Mytilus edulis, with Byssus
We have made use of the terms dorsal and ventral in speaking of the shell of a bivalve, and it is important that these and a few other similar terms be well understood by those who are about to read the descriptions of the animals, or who may desire to describe them themselves. To do this, take a bivalve in your hand, and hold it before you in such a position that the hinge is uppermost, and the siphons turned towards you. The foot of the animal is now pointing in the direction you are looking, and the mouth, situated at the base of the foot, is also directed the same way. You have now placed the shell, and, of course, also the animal, in such a position that its dorsal side is uppermost, the ventral side below, the anterior end turned from you, the posterior (often narrower) end towards you, the right valve on your right, and the left valve on your left. Knowing the exact uses of these few terms you are in a better position to understand the descriptions of bivalves, and to locate the exact situations of the various internal organs named in such descriptions.
A great deal of the internal anatomy of a bivalve mollusc may be made out by easy dissections, and although the structure of the different species varies in several details, the general characteristics of the group are practically the same in all and may be gathered by the examination of a few specimens.
Fig. 131.—A Bivalve Shell
(Tapes virgineana)
a, anterior; p, posterior; l, left valve; r, right valve; u, umbo, on dorsal side
For this purpose the shell should be prised open by means of some flattened but blunt implement, such as the handle of a scalpel, and then, after inserting a piece of cork to keep the valves apart, gently remove the mantle lobe from the valve which is held uppermost with the same implement, being careful to separate it from the shell without doing any damage to the soft structures. Separating the mantle from the shell in this way we meet with one or more hard masses of muscle that are joined very firmly to the latter. These are the adductor muscles that pass directly from valve to valve, and on cutting them through close to the uppermost valve, the latter can be raised so as to expose the body of the animal, mostly hidden by the overlying mantle lobe.
Before raising the upper mantle lobe we observe the heart, on the dorsal margin of the body, near the hinge of the shell, situated in a transparent cavity (the pericardium) containing a colourless fluid. It consists of at least two cavities—a thick-walled ventricle and a thin-walled auricle, and its slow pulsations may be watched with or without the use of a hand lens. On opening the pericardium the heart is still better seen, and if we carefully cut into the thick-walled ventricle we find a tube running completely through its cavity. This is the rectum—the last part of the digestive tube, that commences at the mouth, and terminates in a cavity at the posterior end communicating with the exhalent siphon.
After noting the nature and position of the one or two adductor muscles previously cut through, we turn the upper mantle lobe upwards, laying it back over the hinge of the shell, cutting it through at the bases of the siphons if we find it is united with the opposite lobe at those points; or, if not united, we observe two points at which the lobes touch each other in order to form the siphonal openings.
Several organs are now exposed to view. The lower mantle lobe is seen in close contact with the valve below it, and if we touch its edge we shall probably observe that it is retracted slightly by the contraction of its own muscular fibres. The tip of the foot is also seen projecting towards the anterior end, its base being hidden between the two sets of plate-like gills that extend along the length of the body. On touching the tip of the foot we find it retract by the contraction of the muscular fibres of which it is composed, aided, perhaps, by the action of one or more retractor pedis muscles with which it is supplied. On raising the upper gill-plates we may observe the dark colour of the digestive gland (liver) at the base of the foot, and also see two or more tentacles or labial palpi on the anterior side of the same.
Between the labial palpi is the mouth, which leads into the stomach by a short, wide tube, and then into a convoluted tube which finally passes through the heart, and terminates near the exhalent siphon as above described. The whole length of this tube may be followed by careful dissection, its direction being determined at short intervals by probing it with a bristle that has been tipped with a little melted sealing wax. It will be seen to wind through the base of the foot, surrounded through the greater part of its course by the digestive gland, from which a digestive fluid enters it through small ducts.
The diagram on [p. 194] shows the general internal anatomy of a lamellibranch, parts of which have been removed to reveal the underlying structures. The animal lies in its left valve, the right valve, the right mantle lobe, and the right set of gill-plates having been completely dissected away. The whole course of the digestive tube has also been exposed, and the positions of the three nerve ganglia, with their connecting nerve cords, constituting the central portion of the nervous system, are also indicated.
It will be interesting, finally, to learn the direction taken by the water currents which supply the animal with air and food in their course through the system. Passing in through the inhalent siphon, the water immediately enters a large cavity between the mantle lobes. This cavity (the branchial cavity) contains gills, as we have already seen, and also extends to the mouth. The water, urged on by the motion of myriads of minute ciliated cells in the walls of the cavity, passes in part through the digestive tube, and in part around, between, and through the gill plates, which are perforated by numerous holes. After thus completely bathing the gills, and supplying the oxygen necessary for respiration, this latter current passes into a second cavity above the gills, and thence into the exhalent siphon, where it mingles with the fluid from the digestive tube as well as with other excretory matter.
Lamellibranchs are, as a rule, exceedingly prolific, a single individual of some species discharging more than a million ova in one season. The larvæ swim freely in the water, and are provided with eyes that enable them to search for their food, but the eyes always disappear when the young settle down to a more sedentary life. It is true that adult bivalves sometimes possess visual organs, often in the form of conspicuous coloured spots on the edge of the mantle, these, however, are not the same that existed during the larval stage, but are of a more recent development.
Lamellibranchs are classified in various ways by different authorities, the arrangement being based principally on the number and position of the adductor muscles, or on the nature of the gills. For our present purpose we shall look upon them as consisting of two main divisions—the Asiphonida and the Siphonida, the former including those species which do not possess true tubular siphons, the inhalent and exhalent openings being formed merely by the touching of the mantle lobes; and the latter those in which the mantle lobes are more or less united and tubular siphons formed. Each of these divisions contains a number of families, most of which have representatives that inhabit the sea; and we shall now note the principal characteristics by which the more important families are distinguished, and take a few examples of each, starting with the Siphonida.
Examining the rocks that are left exposed at low tide we frequently find them drilled with holes that run vertically from the surface, seldom communicating with each other within, and varying in diameter from less than a quarter of an inch to half an inch or more. Some of these holes are the empty burrows of a boring mollusc, while others still contain the living animal in situ.
The molluscs in question belong to the family Pholadidæ, which contains a number of species that exhibit very remarkable features both as regards structure and habit. The shell is very thin and fragile, but yet composed of hard material, and its surface is relieved by a series of prominent concentric ridges that bear a number of little rasp-like teeth. It gapes at both ends, has neither true hinge nor ligament, and is often strengthened externally by two or more extra or accessory valves. The hinge-plate is a very peculiar structure, for it is reflected over the exterior of the umbones, above which they are supported by about ten thin shelly plates, the whole thus forming a series of chambers. The accessory valves are supported by these bridged structures, and a long, straight, calcareous plate also fills the space along the dorsal side of the shell in some species. The muscular scars and the pallial line are distinctly seen on the inner surface, and a peculiar curved shelly plate projects from under the umbo of each valve.
Fig. 132.—Pholas dactylus
1, ventral aspect, with animal; 2, dorsal side of shell showing accessory valves
The animal inhabiting the shell is somewhat wormlike in general form, and the mantle lobes are united in front—that is at the lower end of the shell as it lies in the burrow—except that an opening is left for the protrusion of the short foot. The siphons are united and much elongated, so that they protrude beyond the mouth of the burrow when the animal is active; the gills are narrow, and extend into the exhalent siphon; and the anterior adductor muscle, being very near the umbones, serves the double purpose of adductor and ligament.
Such are the general distinguishing features of this family, all the species of which burrow into stone or other material. Those more commonly met with on our coasts belong principally to the genus Pholas, and are popularly known as Piddocks.
It was long a puzzle as to how the fragile piddocks could excavate the tubular burrows in which they live, and, since their shells are so thin that it seemed almost impossible for hard stones to be ground away by them, it was suggested that the rocks were excavated by the action of an acid secretion. This, however, would not account for the formation of holes in sandstone and other materials which are insoluble in acids; and, as a matter of fact, no such acid secretion has ever been discovered. The boring is undoubtedly done by the mechanical action of the rasp-like shell, which is rotated backwards and forwards, somewhat after the manner of a brad-awl, though very slowly, by the muscular action of the foot of the animal.
Piddocks are found principally in chalk and limestones, though, as before hinted, they are to be seen in sandstones and other rocks, the material in any case being, of course, softer than the shell that bores it. The largest holes and the largest specimens are to be found in chalk and other soft rocks; while the piddocks that burrow into harder material are unable to excavate to the same extent and are, as a consequence, more stunted in their growth. The burrowing is continued as long as the animal grows, the hole being always kept at such a depth that the shell is completely enclosed; and not only this, for when the rock is soft, and the surface is worn down by the sea, the piddock has to keep pace with this action, as well as to allow for its increase in size.
As a result of the rasping action of the pholas shell on the surrounding rock the space hollowed out becomes more or less clogged with débris. This is ejected at intervals by the sudden contraction of the foot of the animal, which brings the shell quite to the bottom of the burrow, thus causing the water with its sediment to shoot upwards, It is not usually an easy matter to obtain perfect specimens of the pholas by simply pulling them from their burrows, the shells being so thin and fragile, and the mouth of the burrow being often narrower than the widest part of the shell. The best plan is to chip away the rock with the aid of a mallet and chisel, or to break it into pieces with a hammer, thus laying open the burrows so that the molluscs fall from their places.
The Common Piddock (Pholas dactylus) may be identified by the illustrations, and the other members of the family may be recognised at once by the similarity in structure and habit. The principal species are the Little Piddock (P. parva), the shell of which is wider in proportion to the length, with only one accessory valve; and the White Piddock (P. candida), also with a single accessory. In all the above the foot is remarkable for its ice-like transparency.
Fig. 133.—Pholas dactylus, interior of Valve; and Pholadidea with Animal
There is another genus—the Pholadidea—the species of which are very similar to pholas both in structure and habit. The shells are, however, more globular in form, and are marked by a transverse furrow. The gape at the anterior (lower) end is also very wide, and covered over with a hardened plate in the adult. Also, at the posterior (upper) end of the shell is a horny cup through which the siphons protrude, and the latter, which are combined throughout their length, terminate in a disc that is surrounded by a fringe of little radiating appendages.
In the same family are the molluscs popularly known as ship worms, which are so destructive to the woodwork of piers and jetties, or which burrow into masses of floating timber. Some of these, belonging to the genus Xylophaga—a word that signifies ‘wood eaters’—have globular shells with a wide gape in front, and burrow into floating wood, nearly always in a direction across the grain. The burrows are about an inch deep, and are lined with a calcareous deposit. The siphons, combined except at the ends, are slender and retractile; and the foot, which is thick, is capable of considerable extension.
Fig. 134.—The Ship Worm
Fig. 135.—1. Teredo navalis. 2. Teredo norvegica
Other ship worms belong to the genus Teredo, and are very similar in general characters. The shell is small and globular, with a wide gape at both ends, and consists of two three-lobed valves with concentric furrows. It is so small in proportion to the size of the animal that it encloses but a small portion of the body, and lies at the bottom of the burrow, which is of considerable length—often from one to two feet. The animal is very wormlike in form; and although the shell is so small, yet all the internal organs are enclosed by it. The mantle lobes are united in front, except where the sucker-like foot passes through them; the gills are long and narrow, and extend into the siphonal tube; and the two very long siphons are united almost throughout their length. It is also interesting to note that in these animals the rectum does not pass through the heart, as it does in nearly all molluscs, and that a pair of horny or calcareous ‘styles’ or ‘pallets’ project from the place where the two siphonal tubes begin to diverge.
Several species of Teredo are to be met with on our coasts, but they are so similar in general structure that the above brief description applies almost equally well to all.
Other boring molluscs frequent the British shores, but they belong to quite a distinct family called the Gastrochænidæ because their shells gape widely on the ventral side. Their valves are equal in size and very thin, the hinge has no teeth and the pallial line is sinuated. The margins of the mantle lobes are thickened and united except where a small aperture is left for the protrusion of the finger-like foot. The siphons are very long and retractile, and the gills extend into the inhalent tube. These animals burrow into mud, shells, or stone, often dwelling together in such numbers that their galleries cross one another and form a most intricate network, and the different species are to be found from low-water mark to a depth of a hundred fathoms or more.
Fig. 136.—Gastrochæna modiolina
1, Animal in shell; 2, shell; 3, cell
The British species belong to two genera—the typical genus Gastrochæna, and the Saxicava or stone-borers.
The former contains the Common Flask shell (G. modiolina) which burrows into limestone and shells, in the latter case passing generally through the shells into the ground below, and completing its home by cementing together any fragments of hard material that come in its way into a flask-shaped cell. The opening of the burrow is shaped like an hour-glass, the two expansions serving for the protrusion of the siphonal tubes, and the neck of the flask-shaped abode is usually lined with a calcareous layer that projects slightly to afford further protection to the extended siphons. Although this species is very common on some parts of our coast, it is seldom obtained without the aid of a dredge, for it usually lives at a depth of from five to ten fathoms; and when found it is generally no easy matter to extricate them from their holes, to the sides of which they often cement their shells.
The genus Saxicava contains a few species that drill holes, often several inches deep, in shells and stone, and frequently do great damage to breakwaters and other artificial structures. The foot is usually provided with a byssus by which the animal fixes itself to a little projection on the side of its burrow. The species are to be found from low-water mark to a depth of one hundred fathoms or more.
The next family, named Anatinidæ, contains a number of molluscs that burrow in mud or sand or live in seclusion in the crevices of rocks. Their shells are thin, with a granulated outer surface, and the valves are united by a thin external ligament. The inner surface is pearly, the pallial line usually sinuated, and both valves are pitted for the reception of the somewhat stout internal cartilage. The mantle lobes are united, as are also the siphons to a greater or lesser extent; and there is only one gill on each side.
Fig. 137.—1. Thracia phaseolina. 2. Thracia pubescens, showing Pallial Line
Some of the common species of this family are popularly known as Lantern shells, and perhaps the most familiar of these is Thracia phaseolina, the specific name of which is given on account of a fancied resemblance of the shell to a bean. The shell is very fragile, and although large numbers may often be seen stranded on sandy beaches, but few of them are perfect specimens.
The family Myacidæ may be recognised by the thick, strong, opaque shells, usually gaping at the posterior end; the wrinkled epidermis which covers the whole or part of the shell; and the united siphons, which are more or less retractile. The mantle cavity is also closed with the exception of a small hole left for the protrusion of the small foot. The pallial line of the shell is sinuated.
Fig. 138.—1. Mya truncata. 2. Interior of Shell. 3. Mya arenaria. 4. Corbula nucleus
In the above illustration we represent the Common Gaper (Mya arenaria), which burrows to a considerable depth in the sand or mud, especially in the estuaries of rivers, from between the tide-marks to a depth of twenty fathoms or more. It may be readily distinguished, in common with the other species of the same genus, by the characteristic wrinkled, membranous tube that encloses its fringed siphons, the membrane being a continuation of the epidermis that extends over the shell. Another characteristic feature of the genus is the large, flat process inside the left valve for the attachment of the internal cartilage. An allied species, Mya truncata, is often found abundantly in company with the above, and may be known by the abruptly squared posterior end.
Other species of the Myacidæ inhabit our shores, including the little Basket shell (Corbula nucleus), the left valve of which is much smaller than the right, which overlaps it. The latter, also, is covered with epidermis, while the former, which is flat, is quite naked.
Fig. 139.—Solen siliqua
The valves have been separated and the mantle divided to expose the large foot
We now come to the interesting family of Razor shells (Solenidæ), specimens of which are washed up on almost every sandy beach, while the living molluscs may be dug out of their burrows at low-water mark. The shells are elongated, gaping at both ends with an external ligament; and the hinge has usually two teeth in one valve and three in the other. The foot of the animal is cylindrical, large and powerful; and the siphons are short and united in the long species, but longer and only partially united in the shorter ones. The gills are long and narrow, and are prolonged into the inhalent siphon.
These molluscs lie vertically in their deep burrows at low-water mark, the opening of the burrow having a form resembling that of a keyhole. While covered with water they occupy the upper portion of their abode, but sink to a depth of a foot or more when the tide goes out. As we walk along the water’s edge at extreme low tide we may observe jets of water that are shot into the air before us. These are produced by the sudden retreat of the ‘Razor-fish’ to the bottom of its burrow when alarmed by the approaching footsteps. Owing to this wariness on the part of the mollusc, and to the considerable depth of its burrow, specimens cannot be obtained by digging without much labour; but if a little salt or some other irritant be dropped into the hole, the animal will soon rise to eject it, and may then be shut out from the lower part of the burrow by sharply driving a spade below it. This is undoubtedly the best method of securing perfect specimens for study or preservation, but fishermen often obtain large numbers, either for food or for bait, by suddenly thrusting a long hook down into the gaping shells, and then pulling them out. This method always does injury to the soft body of the animal, and often damages the shell, but answers the fisherman’s purpose exactly.
We give illustrations of two shells belonging to the typical genus (Solen), including one on [Plate V.]; also a British representative of each of two other genera of the family—Cerati-solen and Solecurtus, the latter of which, as the name implies, contains shorter species.
Fig. 140.—1. Solen ensis. 2. Cerati-solen legumen. 3. Solecurtus candidus
The next family—the Tellinidæ—contains a number of well-known molluscs that burrow into sand or mud, and are enclosed in shells that are often very prettily marked; and although the family includes several genera, all may be recognised by the following general features. The shell is compressed, composed of two equal valves, with little or no gape, and the ligament situated on the shortest side. The central or cardinal teeth never exceed two in number in each valve, and the adductor impressions are round and polished. The mantle is quite open at the anterior end, and its margins are fringed; the foot is flattened and tongue-shaped; and the siphons, which are quite separate, are generally long and slender.
In the typical genus (Tellina), of which we represent two very common British species, the ligament is very prominent, and the slender siphons are often much longer than the shell. The members of this group move very freely, travelling about by means of a broad, flattened foot.
Fig. 141.—Tellinidæ
1. Psammobia ferroensis. 2. Donax anatinus. 3. Tellina crassa. 4. Tellina tenuis. 5. Donax politus
The shells of the genus Psammobia are popularly known as Sunset shells, being prettily marked with radiating bands of pink or other tint, reminding one of the beams of the sun when setting in a cloudy sky. In these, too, the ligament is very prominent, and the shell gapes slightly at both ends.
The same family contains the pretty little Wedge shells, which are so called on account of their triangular form, and constitute the genus Donax. These shells, which are seldom much over an inch long, are very common on some of our sandy beaches, being washed up in considerable numbers after the animals have died, but the specimens are seldom perfect. The molluscs themselves are burrowers, and live in the sand, at and just below low-water mark; and, as they usually burrow to a depth of only a few inches, are easily obtained alive.
The shells are rather thin, closed at both ends, blunt and rounded at the anterior end, but straight and more pointed at the shorter posterior end; and the margins of the valves are very finely grooved in such a manner as to resemble the milling of a coin. Each valve has two central hinge teeth, with one long lateral tooth on each side; and the ligament is external and prominent. The lobes of the mantle are fringed; the siphons are separate and diverging, but shorter and thicker than in most of the other Tellinidæ, and the foot is comparatively large, flattened, and pointed.
The genus contains many species, the commonest being, perhaps, D. anatinus, the colour of which is yellowish, banded with brown, and marked by a number of radiating white lines. This colour, however, is due entirely to the thin, shining epidermis that completely covers the valves; and if this is rubbed off the shell itself will exhibit a pale pinkish tint. Another common species (D. politus) may be recognised by the broad patch of white running from the hinge to the margin, on the posterior side of the middle of each valve.
The family Mactridæ contains some British shells popularly known as Trough shells, and the family name itself is derived from the word mactra, which signifies a kneading trough. In this group the shells are all more or less triangular in form, with the valves equal, and are either closed or very slightly gaping. The ligament, perhaps more correctly designated the cartilage, is generally internal, and contained in a deep triangular hollow; and the shell is covered with epidermis. The mantle of the animal is open in front, and the siphonal tubes are united and fringed. The foot is usually large and flattened.
The typical genus, Mactra, contains some common molluscs that bury themselves just beneath the surface of sandy beaches; and these are so abundant in some parts of Great Britain that they are used largely for feeding pigs. Some of the mactras are remarkable for the great power and extensibility of the foot, which, in some cases, is used so vigorously that the animal turns itself quickly over, or even leaps on the ground.
Our example of this genus is M. stultorum, which is a very common object of the shore. Its colour is very variable, usually some shade of grey or brown, and marked by radiating white lines.
The Otter shells (Lutraria), of which we figure one species, are much like the Mactræ in structure, and are usually included in the same family, but in some respects they resemble the Myacidæ or Gapers. The shell is oblong rather than triangular, and gapes at both ends; and the animal buries itself deep in sand or mud, principally in the estuaries of rivers, from low-water mark to a depth of about ten fathoms. The shells are not very common objects of the shore, for they are found only in muddy places, and those of the commonest species (L. elliptica) are too large and heavy to be washed ashore in the sheltered estuaries where they abound.
Fig. 142.—1. Lutraria elliptica. 2. Part of the Hinge of Lutraria, showing the Cartilage Pit. 3. Macra stultorum. 4. Interior of same showing Pallial Line
We now leave the burrowers, to consider a family of molluscs that move about somewhat freely by means of a flattened tongue-shaped foot, and which only rarely fix themselves in any way. The shells of the group are popularly known as Venus shells, probably on account of the beauty of some of the species, and the family in question as the Veneridæ.
The shells of the various species are usually of a graceful oval or oblong form, frequently marked by chevron-shaped lines in pretty colours, and distinctly grooved along the lines of growth. The ligament is external, the hinge has usually three diverging teeth in each valve, and the pallial line is sinuated.
The principal genus is Venus, in which the shells are ovate in form, thick, and smooth, and the margins of the valves are minutely crenulated. The genus is a very large one, and contains several British species, two of which we represent in the accompanying illustrations.
Allied to these is the larger but pretty shell Cytherea chione, which inhabits deep water off the southern coasts, to about one hundred and fifty fathoms. It is much like the Venus shells in form, but the margins are not crenulated.
Fig. 143.—Veneridæ
1. Venus fasciata. 2. Venus striatula. 3. Tapes virgineana. 4. Tapes aurea
The same family (Veneridæ) contains the large genus Tapes, so called because many of its shells are marked in such a manner as to recall the patterns of tapestry. The general form of these shells is oblong, and the margins are quite smooth. They are frequently washed up on the beach, especially during storms, but the animals may be found alive at low water, buried in sand, or hiding in the crevices of rocks or among the roots of the larger sea weeds. The mantle is open at the anterior end, and the siphons are either quite distinct or only partly united.
Some of the shells are very prettily coloured. One (T. aurea) receives its name from the yellow ground, which is variously marked by deeper tints; another (T. decussata) is so called on account of the cross grooves with which the shell is sculptured; and a third (T. virgineana), which inhabits the muddy bottoms of deep water, is prettily marked by radiating bands that run from the umbones to the ventral margins.
We now come to the family Cyprinidæ, in which the shell is regular in form, oval or elongated; and the valves, which are equal in size, are thick and solid, and fit closely. The teeth are beautifully formed, the central ones numbering from one to three in each valve, and the pallial line is not sinuated. The mantle lobes are united on the posterior side by means of a kind of curtain that is pierced by two siphonal openings. There are two gills on each side, united posteriorly, and the foot is tongue-shaped and thick.
The typical genus—Cyprina—contains a large mollusc (C. islandica), which is moderately common round our shores, especially in the north, but is not often seen above low-water mark, except when washed up by storms. The shell is oval and thick, with the umbones prominent and turned towards the posterior side, and the ligament is strong and prominent. It is entirely covered with a thick epidermis, of a rich brown colour, often exhibiting a fine silky gloss, especially near the margins. The interior of the shell is white, and the adductor impressions oval and polished.
The same family includes some smaller shells that inhabit deep water, and are therefore not commonly seen on the beach. Among these are two species of the genus Astarte, one of which is deeply furrowed in a direction parallel with the margins; also Circe minima, which seldom exceeds half an inch in length. Although so small compared with Cyprina, these shells may be identified by their clothing of epidermis, together with the family characteristics given above.
The Cyprinidæ also contains the interesting Heart Cockle (Isocardia cor), the form of which is so characteristic that identification is easy. The heart-shaped shell is thick and strong, and is swollen out in such a manner that the umbones are wide apart. These latter are also curved into a spiral form, and the ligament between them is prominent. The colour of the shell is variable, the epidermis being of any shade from a yellow to a dark brown. The foot is small and pointed, and the siphons fringed.
The Heart Cockle burrows in sand by means of its foot, going down just far enough to bury the whole of its shell, and always leaving its siphons exposed at the surface. It inhabits deep water, and is not likely to be obtained without the use of the dredge or trawl.
Fig. 144.—Cyprinidæ
1. Cyprina islandica. 2. Teeth of Cyprina. 3. Astarte compressa. 4. Circe minima. 5. Isocardia cor
The molluscs of the family Lucinidæ are found principally in tropical and sub-tropical seas, ranging from the shore to a very great depth, but a few are moderately common in our own waters. They are closely allied to the Cyprinidæ, but the shell is round rather than oval, and is obliquely grooved inside. The mantle lobes of the animal are not united on the ventral side, but at the posterior end they are continuous, except where they form one or two siphonal openings. The foot is long and of almost the same thickness throughout when extended; and the gills, numbering either one or two on each side, are large and thick. In all the members of this family, as in the last, the pallial line of the shell is simple. None of the shells are really common objects of our shores, since the animals inhabit deep water, some of them moving about freely on the bottom, while others moor themselves by means of a byssus.
We shall take only one example of the family—Galeomma Turtoni—the generic name of which means ‘weasel eye.’ This pretty little mollusc may be found on our southern coasts, where it often moors itself to the rocks or weeds by means of its silken byssus; or, having broken itself away from its temporary place of rest, creeps freely on the bottom by a long, flattened foot, applied closely to the surface over which it travels, and used much in the same way as the broad foot of a snail or whelk, its valves being all the time spread out nearly in the same plane.
Fig. 145.—Galeomma Turtoni
The shell itself is oval, with central umbones, and is covered with a thick epidermis. The mantle lobes are united behind, where they form a single siphonal opening; and the margins are double, with a row of eye-like spots on the inner edge of each.
The true Cockles, some few species of which are known to almost every one, constitute the family Cardiadæ, so called on account of the cordate or heart-shaped form of the shell as viewed from the anterior or posterior side. The shell is regular, or nearly so, and the valves, which are equal, are ornamented with prominent rays that run from the umbones to the margin. The ligament is short, strong and prominent, and the valves fit closely by the interlocking of their crenulated margins, or gape slightly on the posterior side. There are two central teeth in each valve, and a long lateral tooth both on the anterior and posterior sides. The mantle lobes are open in front, with the margins plaited, and the siphons, which are usually short, are provided with a number of little tentacles. The foot is large and powerful, and is usually curved into the form of a sickle.
Fig. 146.—1. Cardium pygmæum. 2. Cardium fasciatum. 3. Cardium rusticum
Although the general nature of the common edible cockle (Cardium edule) is so well known even to the inhabitants of inland towns that a description may seem out of place here, yet it is possible that but few of our readers have ever taken the trouble to place the animal in a vessel of sea water, either obtained direct from the sea or artificially prepared, for the purpose of studying its movements or other habits; and it will be well to remember that this and several other species of edible molluscs which reach our towns alive may be very conveniently studied at home, and often at times and seasons when work at the sea-side is undesirable or impossible.
The edible species referred to lives in banks of sand or mud, buried just below the surface, and frequently in spots that are exposed for several hours between the tides. They are usually obtained by means of a rake similar to that used in our gardens.
Fig. 147.—Cardium aculeatum
On the coasts of Devon and Cornwall we find a much larger species, also valued as an article of diet, and known locally as the Prickly Cockle (C. aculeatum). Its shell is beautifully formed, the rays being very prominent, each bearing a number of calcareous spines arranged in a single row. We give an illustration of this species, together with two sketches to show the nature of the teeth of the shell.
In addition to the two species named, we have the red-footed, C. rusticum, which can suddenly turn itself over by the action of its powerful pedal organ; the Banded Cockle (C. fasciatum), a very small species distinguished by the brown bands of the shell; and a still smaller one (C. pygmæum), with a triangular shell, occurring on the Dorset and Devon coasts (fig. 146).
Passing now to the Asiphonida, we deal first with the family Arcadæ. These include a number of shells which, though very variable in general form and appearance, may all be recognised by the long row of similar comb-like teeth that form the hinge. The shells of this group are regular in form, with equal valves, and are covered with epidermis. The mantle of the animal is open, the gills are united by a membrane behind, and the foot is large, curved, and grooved.
Fig. 148.—Pectunculus glycimeris, with portion of Valve showing Teeth, and Arca tetragona
One of the prettiest shells in the family is Pectunculus glycimeris, which reaches a length of about two inches. The shell is grooved in the direction of the lines of growth, and there are also very delicate striations running radially from umbones to margin; and the ground colour of white or pale yellowish is beautifully mottled with reddish brown. We give a figure of this species, together with a drawing of the peculiar and characteristic teeth, but a more typical shell of this family may be seen in the Noah’s Ark (Arca tetragona). This shell is almost quadrate in form, swollen, and strongly ribbed. The hinge is straight, with many comb-like teeth—increasing in number with the age of the shell; and the umbones are separated by a diamond-shaped ligament. The foot of the animal is heeled—that is, it has a creeping surface that extends backwards as well as forwards; the mantle is furnished with minute eyes (ocelli), and the animal has two distinct hearts. We give a figure of this peculiar shell, and the other British members of the same genus, though varying more or less in form, may be recognised at once by the same general characteristics.
In the same family we have the small nutshells (genus Nucula), which are often dredged up from deep water in large numbers; and the elongated shells of the genus Leda, also inhabitants of deep water; and, as before stated, the affinities of all may be readily established by the characteristic nature of the teeth.
We now pass on to the family of Mussels (Mytilidæ), of which the common Edible Mussel (Mytilus edulis) is a typical species. In this interesting group the shell is oval or elongated, with equal valves, and is covered with a dark-coloured epidermis which is often distinctly fibrous in structure. The umbones are at the anterior end of the shell, which end is usually very narrow and pointed, while the posterior is broad and rounded. The hinge has small teeth or none, and the ligament, which is long, is internal. The shells of mussels consist of two distinct layers; on the inner, which is often of a most beautiful pearly lustre, may be traced the simple pallial line and the impressions of the small anterior and the large posterior muscles.
The mantle lobes of the animal are united only at a point between the two siphonal openings. There are two elongated gills on either side, and the foot is thick and more or less grooved.
Fig. 149.—Mytilus edulis
Mussels inhabit salt, brackish, and fresh waters, generally attaching themselves by means of a silken byssus, but sometimes concealing themselves in ready-made holes, or in burrows of their own; and some even hide themselves in a nest which they prepare by binding together fragments of shells or sand.
The edible mussel, which forms such an important article of diet, especially among the poorer classes in our large towns, may be easily distinguished from similar species of another genus by the very pointed umbones, and the coarse and strong fibrous byssus by which it clings to any solid object. It is found most abundantly on muddy coasts, and on mud banks in the estuaries of rivers, generally in such situations as are uncovered at low tide. The fry abound just below low-water level, and grow so rapidly that they reach their full size in a single year.
It is well known that a diet of mussels occasionally produces very unpleasant and even dangerous symptoms in the consumer, and this result has been attributed to the action of a particular organ of the animal which has not been carefully removed before eating. This, however, is not the case, as proved by the fact that the eating of these edibles is usually perfectly safe when no such precautions have been taken. It is highly probable that the deleterious character referred to is due to a disease which sometimes attacks the mussels themselves, but the exact nature of this has not been thoroughly made out.
Fig. 150.—1. Modiola modiolus. 2. Modiola tulipa. 3. Crenella discors
There is another genus (Modiola) containing several species commonly known as Horse Mussels, and these may be distinguished from Mytilus by their habit of burrowing, or of constructing a nest by spinning together various fragments. The shell, also, is more oblong in form, and much swollen near the anterior end; and the umbones are not so pointed. The epidermis covering the shell is of fibrous structure, and often extends beyond the edges of the valves in the form of a fringe.
Several species of Horse Mussels inhabit our shores, from low-water mark to a depth of fifty fathoms, but none of them is used for food. The commonest species is Modiola modiolus, which has a particularly strong byssus, and its fibres generally bind together such a number of stones &c. that the shell is completely hidden in the entangled mass. Other British species include M. barbata, so called on account of the peculiar fringed threads of the epidermis; M. phaseolina, in which the epidermis threads are not fringed; and M. tulipa, named from the streaks of crimson or purple that radiate from the umbones of the shell and remind us of the colouring of the tulip flower.
An allied sub-genus (Crenella) includes a few small British molluscs the shells of which are crenulated on the dorsal margin behind the ligament. The shells are short and swollen, and lined by a brilliant pearly layer. One species (C. discors) is pale green, with radiating lines from umbo to margin. It is common on many of our shores, but is not easily found, as it hides at or below low water mark, in a nest formed by binding together small stones. Other species, one of which is black, are less abundant, and are not readily obtained except by the use of the dredge.
Before leaving this family we must refer to the remarkable Dreissena polymorpha, sometimes called the Chambered Mussel, on account of the chamber which is formed in the beak of the shell by means of a pearly plate that stretches across it. This animal is not indigenous to Britain, but was introduced from the East by trading vessels, either attached by its silken byssus to timber that had been left floating in water previous to being shipped, or to the bottoms of the ships. It seems to thrive almost equally well in salt, brackish, and fresh waters, and has spread very rapidly since its introduction. It is more commonly found, however, in docks, canals, and rivers, and is on that account usually described with the fresh-water species.
Fig. 151.—Dreissena polymorpha
The form of the shell is very similar to that of Mytilus, but has no internal pearly layer, and the valves are bluntly keeled. The mantle is closed, the siphons short, and the foot small.
Our next family—the Aviculidæ—contains those shells that are distinguished by peculiar flat processes on each side of the umbones, one of which, the posterior, is generally wing-like in form. They are popularly known as Wing Shells, and the family includes the so-called Pearl Oysters. Most of the species are natives of tropical seas, but several are common on our own shores.
Fig. 152.—Avicula, and Pinna pectinata
One species of the typical genus is sometimes found off the coasts of Cornwall and Devon. The shell is very oblique, and the valves are unequal, the right one, on which the animal rests, being somewhat smaller than the left; and the epidermis is very scanty. The hinge is long and straight, without teeth, and the cartilage is contained in grooves. The interior of the shell is pearly. The posterior adductor impression is large, and not far from the middle of the shell, while the anterior, which is small, is close to the umbones. The mantle of the animal is open, and the margins of the lobes fringed; and the small foot spins a powerful byssus.
Most of the British species of the family belong to the genus Pinna, so called on account of the fins or wings on the dorsal side of the shell. In this group the shell is more or less wedge-shaped, with equal valves, and the umbones are quite at the anterior end, while it is blunted and gaping at the other end. The hinge has no teeth. The margins of the mantle are doubly fringed, and the byssus is extremely powerful.
The Common Pinna (P. pectinata) is a very large mollusc, sometimes measuring a foot in length, and is very abundant off the south-west coast, where it moors itself vertically at the bottom of the water with the pointed end buried, and the broad end gaping widely so as to expose its body. It has been stated that fishes are frequently tempted to intrude into the open shell for the purpose of devouring the animal within, and that they are immediately crushed by the sudden closing of the valves, which are pulled together by two large and powerful adductors.
We have already referred to the little Pea Crab that inherits the shell of the Pinna, living permanently in the mantle cavity of the animal.
The last family of the Lamellibranchs is the Ostreidæ or Oysters, of which the edible oyster may be taken as a type. In this group the shells are frequently unequal, and they lie on one side either free or adherent to the surface below them; the hinge is usually without teeth. The mantle is quite open, the gills number two on each side, and the foot is either small or absent.
The Edible Oyster is a type of the typical genus Ostrea, its scientific name being Ostrea edulis; and as this mollusc may be readily obtained at any time, it is a convenient species for the study of the general characteristics of its family. Its shell is irregular in form, and the animal always rests on its left valve, which is convex, while the upper or right valve is either flat or concave. The lower valve is also thicker and laminated in structure, and is attached to the surface on which it rests. On examining the interior we find that the shell is somewhat pearly in appearance, and that the edges of the mantle lobes are finely fringed. The gills, too, are united with each other and with the mantle on the posterior side, thus forming a distinct branchial chamber.
Oysters are found on banks at the depth of several fathoms, where they spawn in early summer, and the fry or spats are collected in large numbers and transferred to artificial beds or tanks, where they are kept in very shallow water so as to be easily obtainable when required for food. It is interesting to note, however, that their growth is slow on these artificial grounds, the full size being attained in about seven years, while, in the natural beds, they are full grown in a little more than half that time.
Fig. 153.—1. Anomia ephippium. 2. Pecten tigris. 3. Pecten, animal in shell
Native oysters—those that are reared on artificial beds—are of course removed as soon as they are ready for the market, but those that live on natural banks are often left undisturbed till their shells are thick with age. The latter, too, are often destroyed in large numbers by the boring sponge ([p. 124]), which so completely undermines the substance of the shell that it finally breaks to pieces.
In the genus Anomia the lower valve is concave, and perforated with a large oval hole very near the hinge, while the upper one is very convex, but the shell is very variable in shape, since the animal sometimes clings permanently to an object, and the shell, during its growth, accommodates itself to the surface of that object. The use of the hole is to allow of the protrusion of a set of muscles which proceed from the upper valve, and give attachment to a plug or button, more or less calcified, by which the animal clings.
One species (A. ephippium), known as the Saddle Oyster, is common on some parts of our coast. It is seldom found on the beach at low water, but the empty shells are often washed up by the waves.
The same family includes the Scallops, which constitute the genus Pecten. In these the shell is nearly round, with ears on each side of the umbones, those on the anterior side being generally much more prominent than the others, and both valves are ornamented by prominent radiating ribs. The shell is often very prettily coloured, and the animal rests on the right valve, which may be distinguished from the left by its greater convexity, and by the presence of a notch under the anterior ear. The hinge is straight, with a very narrow ligament, and the internal cartilage is situated in a central pit.
Plate V.
MOLLUSCS
| 1. Solen ensis | 8. Tellina |
| 2. Trivia Europæa | 9. Capulus hungaricus |
| 3. Trochus umbilicatus | 10. Chrysodomus antiquus |
| 4. Trochus magnus | 11. Buccinum undatum |
| 5. Littorina littorea | 12 & 13 Scalaria communis |
| 6. Littorina rudis | 14. Pecten opercularis |
| 7. Haminea (Bulla) hydatis | 15. Pecten varius |
| 16. Pecten maximus | |
The mantle of the animal is free, with double margins, the inner of which forms a finely fringed curtain all round, and on this curtain are a number of black eyes surrounded by very fine tentacles. The gills are in the form of very thin crescents, and the foot is shaped like a finger.
Although the majority of scallops are inhabitants of tropical seas, several species are to be found off our coasts, where they range from depths of about four to forty fathoms, and the empty shells, often in the most perfect condition, are frequently found on the beach.
The Common Scallop (P. maximus) is largely used as food, and is therefore a common object in the fishmonger’s shop. Its colour is very variable, and the shell has equal ears and about twenty radiating ribs. The Quin (P. opercularis) is also an important article of diet in some parts.
Perhaps the prettiest of the British species is the Variable Scallop (P. varius), so called on account of the very variable colour of the shell, the ground tint of which may be almost anything between a very pale yellow and a dark reddish brown, and this is irregularly patched with some lighter colour. The chief distinguishing features of the species are the spiny projections of the numerous ribs, most prominent near the margin of the valves, and the presence of a permanent byssus, which, in other species, occurs only in the young. Three of the species named above are shown on [Plate V.]
We may also mention the Tiger Scallop (P. tigrinus), the radiating ribs of which are sometimes slightly formed, and which has only one ear in each valve; and P. pusio, in which the adult shell is often greatly altered in form.
It may be noted, in conclusion, that all the species of this genus have the power of swimming rapidly by flapping their valves—a mode of locomotion very common among the bivalves especially during an early stage of their existence.
Before passing on to the univalve molluscs, we must refer briefly to a group of animals that are enclosed in bivalve shells, and which were once included with the Mollusca, but are now made to form quite a distinct group by themselves. We refer to the Brachiopods, at one time very abundant, as proved by the immense number of fossil shells embedded in various stratified rocks, but now represented by only a few living species.
The shells of these animals are commonly known as Lamp Shells, on account of their resemblance to an antique lamp; and although at first sight they bear a general likeness to certain bivalve shells of lamellibranchs, a close examination will show that not only the shell, but also the animal residing within it, are both of a nature very different from that of the molluscs with which they were at one time supposed to be closely related.
Fig. 154.—Terebratulina. The upper figure represents the interior of the Dorsal Valve
The valves of the shell are unequal, and are not placed respectively on the right and left sides of the body of the animal, but rather on the dorsal and ventral or upper and lower sides. The ventral shell is the larger, and is produced into a beak which sometimes has a round hole corresponding in position with the hole for the wick of an antique lamp, and the dorsal or smaller valve is always imperforate. The hinge is a perfect one, the junction of the two valves being so well secured by it that it is impossible to separate them without injury. It is formed by two curved teeth on the margin of the ventral valve that fit into corresponding sockets on the dorsal. A few brachiopods, however, have no hinge, the valves being secured by means of numerous muscles. The hole in the shell serves for the protrusion of a pedicel or foot by means of which the animal is enabled to attach itself.
Two long arms, covered with vibratile cilia, and capable of being folded or coiled, are attached at the sides of the mouth. They are practically processes of the lips, mounted on muscular stalks, and attached to a delicate calcareous loop on the dorsal valve; and serve not only to produce water currents for the conveyance of food to the mouth, but also answer the purpose of gills.
The digestive system of a brachiopod includes an œsophagus that leads into a simply formed stomach round which is a large digestive gland. The heart has only one cavity, but the animal is provided with two smaller and separate organs that assist in the propulsion of the blood, which circulates through numerous blood spaces in the bristly mantle.
About two thousand fossil species of brachiopods are known, extending over a vast range of time; and the living species, numbering less than a hundred, are found from shallow water to the greatest habitable depths.
Since the reader is hardly likely to form any extensive acquaintance with the Brachiopods, we shall illustrate our remarks by the introduction of only one species—the Serpent’s Head Terebratula (Terebratulina caput-serpentis), which is found in deep water in the North Sea. The interior of the dorsal valve, showing the calcareous loop above referred to, is represented in fig. 154, as is also the exterior of the shell, which is finely striated. The latter represents the dorsal aspect of the shell in order to show the hole in the upturned beak of the ventral valve.
Fig. 155.—Under side of the Shell of Natica catena, showing the Umbilicus; and outline of the Shell, showing the Right handed Spiral
We have now to consider the large group of head-bearing molluscs (Cephalophora), the study of which forms a very important part of the work of the sea-side naturalist; and while we deal with the general characteristics of this group, the reader will do well to have before him a few living typical species in order that he may be able to verify as many as possible of the descriptions here given by actual observation. These types may include such creatures as the whelk, periwinkle, and limpet; or if marine species are not at hand at the time, the garden snail, fresh-water snail, and slug will serve the purpose fairly well.
By far the large majority of Cephalopods are enclosed in a single shell, though a few have a rudimentary shell or none at all.
As is the case with the lamellibranchs, the shell is composed of both animal and mineral substance, the latter being a calcareous deposit secreted by the mantle of the animal. The shell is usually spiral in form, as in the whelk, but sometimes conical (limpet) or tubular.
Spiral shells are nearly always dextral or right-handed; that is, if we trace the direction of the spiral from the apex to the mouth, we find that its turns or whorls run in the same direction as the hands of a watch. A few, however, are sinistral, or left-handed, and occasionally we meet with left-handed varieties of those species that are normally of the right-handed type. The cavity of the shell is a single spiral chamber which winds round a central pillar, and each whorl of the shell generally overlaps the preceding one, the two being separated externally by a spiral depression called the suture.
Sometimes the coils of a shell are not close together internally, so that the central column of the spiral is hollow, and opens to the exterior at the base of the shell. In this case the shell is said to be umbilicated, and the opening referred to is the umbilicus. In others the spiral winds round a solid central pillar which is spoken of as the columella.
Fig. 156.—Section of the Shell of the Whelk, showing the Columella
The apex of the shell, sometimes called the nucleus, is the oldest part, and represents what was once the whole. It is generally directed backwards as the animal crawls, and in adult shells is often more or less worn away by constant friction. We speak of the whorls as first, second, third, &c., taking them in the order of their growth, and it will generally be found that the last whorl is much larger than the others, so much so that it contains the greater part of the body of the animal; hence this one is commonly spoken of as the body-whorl, and the others make up the spire of the shell.
The mouth of the shell is of different forms in different species, but in the herbivorous kinds it is usually simple, while in the carnivorous species it is notched or produced. The edge of the mouth (peristome) is formed by an outer lip which is usually sharp in young shells and either thickened, reflected (turned outward), or inflected (turned inward) in adults; also it may be considerably expanded, or ornamented by a fringed margin. The inner lip is that side of the peristome adjacent to the central pillar of the shell.
If we examine the external surface of several different shells, we find that they are usually more or less distinctly furrowed or sculptured, and that they are often marked by lines or bands of a colour different from that of the ground tint. These furrows, lines, or bands sometimes pass directly from the apex, across the various whorls, to the base of the shell, in which case they are said to be longitudinal. If they follow the course of the whorls, they are described as spiral; and if parallel with the peristome, so that they mark the former positions of the mouth of the shell, thus denoting the lines of growth, they are said to be transverse.
Most univalve shells are covered with epidermis, but in some instances the animal, when extended, surrounds the exterior of the shell with its mantle, as do the cowries, and then the outside of the shell is always glazed. Other species keep their shells covered with the mantle, and in these the shell is always colourless.
The body of the head-bearing mollusc is attached to the shell internally by one or more muscles, and if we examine the interior surface we are generally able to distinguish the impressions or scars denoting the points of attachment.
The reader will have observed that the periwinkle, whelk, and other univalves close their shells by a kind of lid when they retract their bodies. This lid is called the operculum, and is constructed of a horny material, often more or less calcified on the exterior, and is attached to the hinder part of the foot. It sometimes fits accurately into the mouth of the shell, but in some species it only partially closes the aperture. The operculum, like the shell itself, often exhibits distinct lines of growth which display the manner in which it was built up. If these lines are concentric we know that the operculum grew by additions on all sides; but if its nucleus is at one edge, and the lines of growth widest apart at the opposite side, the growth must have taken place on one side only. Some, even, are of a spiral form, denoting that the additions were made continuously at one edge, and such opercula may be right-handed or left-handed spirals.
It will be noticed that in the above general description of univalve shells we have introduced a number of technical terms which are printed in italics, and this we have done advisedly, for the employment of these terms is a very great convenience when giving descriptions of individual shells, and we shall use them somewhat liberally in noting the distinguishing characteristics of the families and genera; but before entering into this portion of our work we must briefly note the general features of the bodies of the Cephalophora.
Fig. 157.—Diagram of the Anatomy of the Whelk, the Shell being removed
c, stomach; e, end of intestine; g, gills; h, ventricle of the heart; a, auricle; f, nerve ganglia; b, digestive gland; ft, foot; o, operculum; d, liver
Sometimes these bodies are bilaterally symmetrical, as we have observed is the case with the worms, but more commonly the organs on one side are aborted, while the growth proceeds apace on the opposite side. Thus the animal assumes a spiral form, being coiled towards the aborted side, with the gills and other organs developed on that side only. As a rule this curvature is such that the body takes the form of a right-handed or dextral spiral, as we have already observed in the shells which cover them, the mouth being thus thrown to the right, but sometimes it takes the opposite direction.
When one of these animals is extended and creeping, we observe that it has a distinct head, furnished with a mouth below, and tentacles and eyes above; also, if an aquatic species, the gills are more or less prominent. Further, the exposed portion of the body is covered with a leathery mantle, and the animal creeps on a broad, flattened surface which is called the foot.
The tentacles or feelers are usually retractile, and, when retracted, are turned outside-in. Each one is provided with a muscle that runs from the body internally to the tip; and, by the contraction of this muscle the tentacle is involuted just in the same way as the finger of a glove could be by pulling a string attached to the tip inside. In addition to these tentacles, and the eyes and mouth previously mentioned, the head is furnished with ear-sacs, which are little cavities, filled with fluid containing solid particles, with nerve filaments distributed in the walls.
On the floor of the mouth there is a ribbon, supported on a base of gristle, and covered with numerous minute teeth arranged regularly in rows. The gristle is moved backwards and forwards by means of muscles in such a manner that this ‘lingual ribbon’ acts like a rasp, and is employed in scraping or tearing away portions of the substance on which the animal is feeding. By this action the teeth are gradually worn away in front, but this is of no consequence, for the lingual ribbon is always growing forwards, the worn material being replaced by new growth behind.
Fig. 158.—A portion of the Lingual Ribbon of the Whelk, magnified; and a single row of Teeth on a much larger Scale
b, medial teeth; a and c, lateral teeth
The arrangement and form of the teeth are characteristic and important; and since they afford one of the means by which we may trace the natural affinities of similar species, they will be frequently referred to when dealing with the principles of classification. For this reason the student should be prepared to examine the lingual ribbons of molluscs with the aid of a compound microscope as occasion requires. As a rule the ribbon is easily stripped away from the floor of the mouth; and, if placed in a drop of water and covered with a cover-glass, the teeth are readily observed. Until a little experience has been gained the observations may be confined to some of the larger species, in which the ribbon is both large and easily obtained. In the common whelk, for example, it often measures more than an inch in length.
It is difficult to understand how the univalve mollusc manages to glide along so rapidly and gracefully on its expanded foot when we observe it from above, but the difficulty is cleared away when we see it creeping on the side of a glass aquarium, or when we place it on a sheet of glass and observe its movements from the other side. We then see that the foot is in complete contact with the glass, and that a steady but rapid undulatory movement is produced by the successive expansions and contractions of the disc, brought about, of course, by the action of muscular fibres.
A few of the univalves are viviparous—that is, they produce their young alive; but the majority lay eggs. The eggs are often enclosed in horny cases, some of which may be commonly seen washed up on the beach, or attached to rocks and weeds between the tide-marks. The larvæ are always enclosed in a shell, though they are sometimes wholly or partially concealed by the mantle. The shell is usually closed by an operculum; but as the animal advances in age the shell sometimes disappears altogether, or is reduced to a mere shelly plate, as is the case with the land and marine slugs and sea lemons. The young of the water-breathers always swim about freely by means of a pair of ciliated lobes or fins, but these remain only for a brief period, after which the animal settles to the bottom for a more or less sedentary existence.
Fig. 159.—Egg Cases of the Whelk
The Cephalophora fall naturally into two fairly well-defined groups, which we may describe as the air-breathers and the water-breathers. The former breathe air direct from the atmosphere through an aperture on the right side of the body, the air passing into a pulmonary organ or lung, in the walls of which the bloodvessels ramify, and they include all the land snails and slugs. The latter breathe by gills which are more or less prominent on the sides of the body, and include all the fresh-water snails, as well as the marine species which fall within our special province.
We shall first consider the class Pteropoda or Wing-footed Molluscs, so called from the wing-like appendages that are attached to the side of the mouth, or to the upper side of the foot, which is either very small or altogether wanting.
These Pteropods are in many respects lowly organised as compared with the higher molluscs; and as they spend the whole of their existence in the open sea, they can hardly be considered as falling within the scope of the sea-side naturalist’s work. Yet since their shells are occasionally drifted on to the shore, and because a knowledge of them is essential to the student of the mollusca, we shall briefly note their principal characteristics.
The pteropods are extremely abundant in some seas, occurring in such vast numbers that they discolour the water for miles. They swim about by flapping the pair of wings already referred to. They are known to form an important article of the diet of the whale, and are also devoured in enormous numbers by various sea birds; and they are themselves carnivorous, feeding on various smaller creatures that inhabit the open waters.
Fig. 160.—Pteropods
In appearance they much resemble the young of higher species of molluscs. The nervous system consists of a single ganglion situated below the gullet, and the eyes and tentacles are either rudimentary or absent. The digestive system includes a muscular gizzard provided with teeth for the mastication of food, and a digestive gland or liver for the preparation of a digestive fluid. The heart has two cavities, and respiration is effected by a surface covered with minute cilia. This surface is either quite external or is enclosed in a chamber through which water freely circulates.
The shell is very different from that of a typical head-bearing mollusc, for it generally consists of two glassy, semitransparent plates, situated dorsally and ventrally respectively on the body of the animal, with an opening for the protrusion of the body, and others at the sides for processes of the mantle; and it terminates behind in one or three pointed processes. Sometimes, however, its form is conical or spiral, with or without an operculum. We append illustrations of a few pteropods, selecting for our purpose species that have been found in the Atlantic.
It will have been noticed from the above short description that the pteropod is very unlike the typical Cephalophore as outlined in our general remarks on the group, especially in the symmetrical form of both body and shell and in the total or almost total absence of the foot; and this distinction is so marked that the pteropods are often separated from all the other Cephalophora into a class by themselves, while all the remainder are placed in a separate extensive class called the Gasteropoda, because they creep on the ventral surface of the body, the term signifying stomach-footed.
These gasteropods are divided into four orders: the Nucleobranchiata, in which the respiratory and digestive organs form a nucleus on the posterior part of the back; the Opisthobranchiata, with gills more or less exposed towards the rear of the body; the Pulmonifera, or lung-breathing order; and the Prosobranchiata, in which the gills are situated in advance of the heart. The third order includes all the land snails and slugs, and does not therefore fall within the scope of our work; but the remaining three consist either exclusively or principally of marine species, and will be dealt with in the order in which they are named.
The Nucleobranchs are not really gasteropods in the strictest sense of the term, for they do not creep along by means of their foot, but all swim freely in the open ocean, always at the surface, and sometimes adhere to floating weed by means of a sucker. In fact, the foot of these creatures is greatly modified in accordance with their habits, one part being often expanded into a ventral swimming fin, and provided with a sucking-disc for adhesion, and another produced into a posterior fin for locomotion.
Like the pteropods, the nucleobranchs are purely pelagic, so that we can hardly expect to meet with a specimen on or near the shore; and thus we shall content ourselves with a brief notice of their general characters.
The shell is very variable in size and form, and sometimes even entirely absent. Large-bodied species often possess but a very small shell, while some are able to entirely retract themselves and close the mouth of the shell by an operculum. These animals are generally provided with a large cylindrical proboscis, and the tongue has recurved teeth. The body is usually very transparent, often so much so that the blood may be seen circulating within it, and the nervous system is much more perfectly developed than in the pteropods. The eyes, too, are perfectly formed.
The presence of special breathing organs may seem to be superfluous in such delicate and soft-bodied creatures as these, for it may be supposed that all the oxygen required could be absorbed directly from the water through their soft structures, as is really the case with many aquatic creatures; and as a matter of fact some of the nucleobranchs possess no gills, but others have these organs fully formed.
Passing now to the true gasteropods, we shall first consider the Opisthobranchs, which are commonly known as Sea Slugs and Sea Lemons. Some of these have no shell at all, and even where one exists it is very rudimentary, usually very small and thin, and concealed within the mantle. The gills are either branched and tree-like, or are composed of tufts or bundles of filaments; and, as the name of the order implies, are situated towards the posterior part of the body. They are also retractile, and when the animal is alarmed it will conceal its gills, thus reducing its body to a shapeless, slimy mass, inviting neither to sight nor to touch.
The sea slugs are principally animal feeders, subsisting on small crustaceans, other molluscs, &c.; the food being first reduced by the rasping action of the teeth, and then masticated in a gizzard which is provided internally with horny spines or hard, shelly plates.
It will not be necessary to enumerate all the different families of this order, especially as the species are mostly to be found beyond the tide-marks, and are therefore obtained only with the aid of the dredge; but we shall describe a few of the British species with a view of showing the general characteristics of the animals.
They are usually divided into two sections, those with exposed or naked gills (Nudibranchiata) forming the first, and those in which the gills are covered either by the shell or the mantle (Tectibranchiata) comprising the second.
In the Nudibranchs the shell exists only during the embryonic stage, and the external gills are arranged on the back or along the sides. The tentacles are not employed as organs of touch, but are probably connected only with the sensation of smell, being provided with filaments of the olfactory nerve; the eyes are small dark-coloured spots embedded in the skin behind the tentacles. Various species are to be found on all rocky coasts, where they range from low-water mark to a depth of fifty or sixty fathoms, but a few are pelagic, living on the surface of floating sea weeds.
It is almost impossible to identify the species of nudibranchs from dead specimens, for the classification of the section is based largely on the arrangement of the gills, which are almost always retracted in the dead animals. This is also the case even with living specimens when disturbed or removed from the water; hence they should always be examined alive in sea water, while the animals are extended and moving.
Fig. 161.—Nudibranchs
1. Doto coronata. 2. Elysia viridis. 3. Proctonotus mucroniferus. 4. Embletonia pulchra
It will be understood from the above statements that special methods will be necessary when it is required to preserve specimens for future study, the gills being always retracted when the animal is killed for this purpose by any rapid process. We have found two methods, however, that are fairly satisfactory in the majority of instances.—Place the living animals in a suitable vessel of sea water, and leave them quite undisturbed till they are fully extended, and then either gradually raise the temperature till they are dead, or introduce into the water, cautiously, a solution of corrosive sublimate. In the latter case a much larger proportion of the sublimate will be required than when used for a similar purpose with freshwater molluscs. When the animals are dead it will be found that their gills are more or less extended, sometimes fully so, and they may then be transferred to diluted spirit or a two per cent. solution of formaldehyde.
Fig. 162.—Nudibranchs
1. Dendronotus arborescens. 2. Tritonia plebeia. 3. Triopa claviger. 4. Ægirus punctilucens
In fig. 162 we represent four species. Two of these—Triopa claviger and Ægirus punctilucens—belong to the family Doridæ, the members of which are popularly known as Sea Lemons, and are distinguished by the presence of plume-like gills situated on the middle of the back. Another family (Tritoniadæ), characterised by the arrangement of the gills along the sides of the back, and by tentacles that can be retracted into sheaths, is represented by Tritonia plebeia and Dendronotus arborescens in the same figure, and by Doto coronata in fig. 161. The family Æolidæ also have their gills arranged along the sides of the back, but they differ from the last in that their tentacles are not retractile. They include the two species numbered 3 and 4 on fig. 161. The remaining one on fig. 161—Elysia viridis—is a member of the family Phillirhoidæ, characterised by a pair of tentacles on the dorsal side of the head and by the foot being either very narrow or absent, the latter feature denoting that the animals are not adapted for creeping on the bottom. In fact, several of the species of this family swim freely by means of flattened tails.
The Tectibranchs are similar in general structure, but are very different in appearance, inasmuch as the gills, so prominent in the last division, are here covered by the mantle, or by the shell, which is often well developed. The latter is very variable in form, being of a globular, twisted, spiral, or other shape, but is sometimes absent in the adult. In fig. 163 we give a few examples of the shells of British species; and one (Bulla hydatis) is shown on [Plate V].
Fig. 163.—Shells of Tectibranchs
We now pass on to the largest and last order of gasteropods—the Prosobranchiata—so called because the gills are situated in front of the heart. This group is an important one to the sea-side naturalist, since it contains nearly all the univalve molluscs that are common between the tide-marks of our shores, as well as some abundant species that are protected by a shell of several distinct parts. In nearly all of them the abdomen is well developed, and the shell is sufficiently large to cover the whole animal when the latter is retracted; and the gills, which are either pectinated (comb-shaped) or plumed, are lodged in the chamber formed over the head of the animal by the mantle.
The order is often divided into two sections—the Holostomata or Sea Snails, in which the margin of the aperture of the shell is entire, and the Siphonostomata, in which the margin of the mantle is prolonged into a siphon by which water passes into the gill chamber. This division does not seem to be very satisfactory, as the sections are not separated by very prominent natural characteristics, but it becomes convenient on account of the great extent of the order.
In the Holostomata the shell is either spiral, conical, tubular, or composed of several valves, and the spiral forms are usually closed by a horny or shelly operculum of the spiral kind. The head is provided with a proboscis that is generally non-retractile, and the gills usually extend obliquely across the back, or are attached to the right side behind the head.
We shall first consider the lower forms, starting with the family Chitonidæ, the animals of which, as the name implies, are covered with a shell that resembles a coat of mail.
Some of these creatures are very common on our rocky coasts, and yet their nature is such that they are liable to be overlooked by those who are not acquainted with their appearance and habits. The shell is oval or oblong, often so coloured as to closely resemble the rocks and stones over which they crawl; and the animal is so inactive when left exposed by the receding tide, and its flat under surface so closely applied to that on which it rests, that it looks merely like a little convexity of the rock. But after a few have been discovered the eye becomes accustomed to their appearance, and large numbers may be obtained in a short space of time.
The shell will be seen to consist of eight transverse, curved plates, overlapping each other at their edges, and all enclosed in a leathery mantle, which also forms a projecting margin all round. The middle six plates are different from the first and last in that they are grooved in such a manner that each one displays a dorsal and two lateral areas.
The animal holds on tightly to the rocks by its large creeping disc-like foot, but may be removed without injury by forcing a knife-blade under the margin of its shell. When examined it will be found that it has not a well-formed head like the majority of the gasteropods, and both eyes and tentacles are wanting. The gills form a series of lamellæ round the posterior end of the body, between the edge of the foot and the mantle; and it is interesting to note that the Chitons further justify the low position assigned to them among the gasteropods by their possession of a simple, central, tubular heart, similar to that of worms.
Perhaps the commonest of the British species is Chiton cinereus. Its colour is a dull grey, but the ground is variously mottled, often in such a manner as to give it a protective resemblance to its surroundings. C. ruber is the largest of our species: its shell is variously mottled with shades of yellow and brown; C. fascicularis is bristled. Another rather common species (C. lævis) is distinguished by the glossy appearance of the dorsal portion of the shell.
It will have been observed that the chitons differ from the majority of gasteropods in that their shells and bodies are both bilaterally symmetrical, and the same is true of the next family—Dentaliadæ, which derive their name from the tooth-like form of their conical shells. They are popularly known as the Tooth Shells, and although they generally live beyond low-water level, they may sometimes be seen alive on the beach, and the empty shells are often washed up by the waves.
The shells (fig. 165) are curved, and open at both ends, the narrower extremity being the posterior. The mouth is circular, and the outer surface is quite smooth or grooved.
Fig. 164.—Chiton Shells
Fig. 165.—Shells of Dentalium
In these animals, too, the head is imperfectly formed, without eyes or tentacles. The foot is conical and pointed, with two symmetrical side lobes; and the gills, also two in number, are symmetrically disposed. The margin of the mouth is fringed, and the animal is attached to the shell near the posterior end.
The Dentaliadæ are carnivorous, subsisting on minute molluscs, foraminifera, &c., and generally live on sandy or muddy bottoms, in which they sometimes bury themselves.
Our next family includes the familiar Limpets, and is designated Patellidæ on account of the resemblance of the conical shell to a little dish. In these the apex of the cone is not central, but situated more or less towards the anterior; and the muscular impression within is shaped like a horseshoe, with its open end turned to the front.
Unlike the members of the preceding families, the limpets have a well-formed head furnished with both eyes and tentacles, the former situated at the bases of the latter. They have a horny upper jaw, and the tongue, which is very long, is supplied with numerous hooked teeth. The foot is a very large disc, as large as the shell, and the gills consist either of one or two branched plumes, or of a series of lamellæ almost or entirely surrounding the animal between the shell and the margin of the mantle.
The reader has probably experienced the difficulty of detaching a limpet from its hold on the rocks. The tenacity of the grip is not due to the mere adhesive power of the foot itself, but to atmospheric pressure, the effect of which is complete on account of the total exclusion of air from under the disc of the foot; and when we remember that this pressure amounts to fifteen pounds on every square inch of surface, we can readily understand the force required to raise a large limpet from its position.
Fig. 166.—Patellidæ
1. Patella vulgata. 2. P. pellucida. 3. P. athletica. 4. Acmæa testudinalis
The Common Limpet (Patella vulgata) is found on all our rocky coasts between the tide-marks, often at such a level that it is left exposed to the air for eight or nine hours at a time. The apex of the shell of this species is nearly central, and the exterior is sometimes nearly smooth, but more commonly relieved by radiating ribs.
Although the shell itself is not a particularly pretty object, it is often rendered very beautiful and interesting by the various animal and vegetable organisms that settle on it. Those shells that are left dry for hours together are commonly adorned with clusters of small acorn barnacles, while the limpets that have found a home in a rock pool and are perpetually covered with water, often resemble little moving gardens in which grow beautiful tufts of corallines or other weeds, as well as polyzoa and other animal forms.
It appears that limpets are not great travellers, the appearance of the rock from which they have been removed being such as to point to a very long period of rest. Those on hard rocks are generally situated on a smooth surface just the size of the shell and generally worn slightly below the surrounding level by the constant friction of the shell; while others that have settled on very rugged spots have their cones adapted to the irregular surface. It has been suggested that the animals make occasional short excursions from their chosen spot, but return again to it; and whether or not this is the case, it is evident that they frequently keep to one small spot for a considerable length of time.
Limpets on chalk and other soft rocks are sometimes in circular pits so deep that even the apex of the shell is below the general level around; and though it is possible that the abrasion is produced entirely by the friction of the shell as the animal turns, yet, in the case of chalk, the action may be partly due to the carbonic acid gas given off by the animal as a product of respiration, for it is a well-known chemical fact that this gas, in solution, has the power of dissolving calcareous material.
The other British Limpets include P. pellucida, which lives on the fronds and stalks of the tangle, the form of the shell varying according to that of the surface on which it rests; also the Horse Limpet (P. athletica), the bold radiating ribs of which are irregularly notched; and Acmæa testudinalis—the Tortoiseshell Limpet, with reddish-brown mottlings on the exterior, and a dark-brown patch at the apex within. The last-named species lives principally on sea weeds, and has a single pectinated gill in the cavity between foot and mantle, which is protruded on the right side when the animal is extended. This latter feature is interesting since it shows a tendency to that one-sided development already referred to as characteristic of the typical gasteropod, resulting in the spiral form of the adult.
In the limpets the lingual ribbon is proportionately long, and is easily removed for examination. In P. vulgata it may exceed an inch in length, and the teeth are arranged in rows each of which contains four central, with laterals on either side, while in Acmæa there are only three laterals on each side of the central line.
Other so-called limpets belong to separate families. Thus we have the Cup-and-Saucer Limpet and the Bonnet Limpet in the Calyptræidæ. Both these differ from Patella in that the apices of their shells show a tendency to assume a spiral form, thus denoting a somewhat closer relationship to the more advanced univalves. They have distinct heads, with prolonged muzzles, and well-formed antennæ and eyes. The teeth of the lingual ribbon are single, with dentated laterals on either side.
Fig. 167.—Calyptræa sinensis
The Cup-and-saucer Limpet (Calyptræa sinensis) is so called on account of a curved plate that projects from the interior of the shell, at the apex; and though this plate takes the form of a half-cup rather than of a cup, the whole shell has suggested the popular name, while the generic name is derived from calyptra, which signifies a cap. This mollusc is occasionally found among stones at low tide, but usually lives beyond this line, thus necessitating the use of a dredge. The Bonnet Limpet (Pileopsis hungaricus) is of similar structure and habit, but the nucleus of the shell is a more decided spiral (see [Plate V.]). Both these animals adhere to stones and rocks, and, like the common limpet, seldom or never move from their selected sites; hence their shells are variable in form, being adapted to the rock below, and the movements of the shell often cause a little hollow to be scooped out of the softer materials.
Yet other limpets belong to the next family Fissurellidæ, which is characterised by a perforation or a notch in the shell. In these, too, the shell is conical, with a tendency to assume the spiral form, but the curve of the nucleus, which is always apparent in the young shell, frequently disappears as the growth proceeds.
Fig. 168.—Fissurellidæ
1. Puncturella noachina. 2. Emarginula reticulata. 3. Fissurella reticulata
In the Keyhole Limpet (Fissurella reticulata) which is found chiefly on our southern shores, the perforation is at the summit of the shell; but as the animal grows the hole increases in size, encroaching on the curved nucleus until the latter quite disappears. In the genus Puncturella the perforation is just in front of the recurved apex, and is surrounded by a rim internally; while in the Notched Limpets (genus Emarginula) it is represented by a fissure on the anterior margin of the cone. In all, however, the hole or notch serves the same purpose, for it is the means by which water enters the siphon.
Fig. 169.—Haliotis
It is doubtful whether we ought to claim the beautiful Ear shell (Haliotis tuberculata) as one of our own, but it is generally included among the British molluscs on the ground that it is abundant on the coast of the Channel Islands, where it is called the Omar; and it is certainly too beautiful an object to be excluded from the British species without ample cause.
It belongs to the family Haliotidæ, and our illustration will show that the shell is less elevated than that of limpets, and that the spire, though not prominent, is a fairly well-formed spiral. All along the outer lip of the very large aperture is a series of perforations, occupying the summit of a prominent, spiral ridge, and becoming gradually smaller and smaller towards the spire. The whole shell is pearly in structure, and displays a great variety of rich colouring. It is used largely for inlaying and other ornamental purposes, and for making the so-called pearl buttons. The animal is used largely as an article of food in the Channel Islands, but it is of so tough a nature that it requires a vigorous beating previously to being cooked.
Fig. 170.—Ianthina fragilis
The same family contains the beautiful violet Ianthina, which also is not a British species, but a free-swimming oceanic snail. It is, however, occasionally drifted to our shores, though generally in an imperfect condition. In the Atlantic and the Mediterranean it sometimes abounds in such multitudes as to distinctly colour the surface of the sea.
It will be seen that the shell is round, with a well-formed spiral. The spire is white, but the base is of a deep violet colour. The animal is very remarkable in some respects. In the first place, though it has pedicels similar to those on which the eyes of the higher univalves are placed, yet it has no eyes. Then the foot, which is in itself small, secretes a float or raft so large that it cannot be retracted into the shell, with numerous air vesicles to render it light, and the egg-capsules of the animal are attached to the underside of this. The animal has no power of sinking, but lives exclusively at the surface; and, when disturbed, it exudes a violet fluid that colours the surrounding water. It is apparently the only gasteropod that lives in the open sea and has a large and well-formed spiral shell.
Passing now to the family Turbinidæ we meet with turbinated or pyramidal shells that are of a brilliant pearly lustre within, and frequently without also when the epidermis is removed. The animals inhabiting them have well-formed heads with a short muzzle, long and slender tentacles, and eyes mounted on peduncles. The sides are ornamented with fringed lobes and several tentacle-like filaments, and the aperture of the shell is closed, when the animal is retracted, by a spiral operculum. They are all vegetable feeders; and, as is usual with the plant-eating molluscs, the teeth on the lateral portions of the lingual ribbon are very numerous.
We have a few common species belonging to this group, mostly members of the typical genus Trochus and commonly known as Top Shells. In these the shell is a pyramid formed of numerous flat whorls, with an oblique and rhomboidal aperture. Of the three species figured (including two on Plate V.) T. umbilicatus and the Large Top (T. magnus) are umbilicated, the umbilicus being very large in the latter; and the former is characterised by the zigzag greyish or reddish markings that run radially across the whorls. The other (T. zizyphinus) is usually of a yellowish or pink colour and has no umbilicus.
The same family contains the pretty little Pheasant Shell (Phasianella pullas), which is richly coloured with red, brown, and yellow on a light ground; and Adeorbis subcarinatus, shown in the same group.
Fig. 171.—1. Trochus zizyphinus. 2. Under side of Shell. 3. Trochus magnus. 4. Adeorbis subcarinatus
The well-known Periwinkle (Littorina littorea) and the species to the right of it on [Plate V.], belong to the family Littorinidæ, the members of which are similar in structure and habit to Trochus, but the shell is usually more depressed, and is never pearly. The shell of the Periwinkle is thick, having but few whorls, and is not umbilicated; and the lingual ribbon, which is coiled up on the gullet, contains no less than about five hundred rows of teeth; but only a little more than twenty of these rows are in action at any one time, the remainder being a reserve stock to come into active service as the ribbon grows forward. In the genus Lacuna there is a narrow umbilicus, and the aperture of the shell is semilunar in form; and the species of Rissoa are very small, with white or horny shells, much more pointed and having more whorls than those of the Littorina.
Fig. 172.—Rissoa labiosa and Lacuna pallidula
Our next illustration shows three shells of the family Turritellidæ, so named from the resemblance of the shells to a tower or spire. The form indeed is so characteristic that they can hardly be mistaken. It will be seen that Turritella communis is striated spirally, while the surface of Scalaria communis (Plate V.) is relieved by strongly marked transverse ribs. Both these species are very common, and the latter is peculiar for its power of ejecting a dark purple fluid when molested. The other representative of the family—Cæcum trachea—has a shell something like that of Dentalium (p. 238), being cylindrical and tubular, but it differs in being closed at one end.
Fig. 173.—Section of Shell of Turritella
Fig. 174.—Turritella communis and Cæcum trachea
In the succeeding shells, of the family Cerithiadæ, the spire is also considerably produced, so much so that some of the species closely resemble the Turret shells, but they are distinguished by usually having an expanded lip, at least in the adult form; and the mouth is channelled in front, and sometimes also behind. The animals of the group have short muzzles that are not retractile, the tentacles are wide apart, and the eyes are mounted on short pedicels. The median teeth are arranged in a single row, with three laterals on either side of each.
Fig. 175.—Cerithium reticulatum and Aporrhais pes-pelicani
Cerithium reticulatum receives its generic name from its appearance to a small horn, and the specific name refers to the netted appearance of its surface due to the presence of numerous little tubercles arranged in rows—a feature that serves to distinguish it from the small Turret shells. It is a common shell, as is also the other representative of the family illustrated, but the latter is rendered conspicuous by the enormously expanded lip that has earned for it the popular name of Spout Shell. Its scientific name is Aporrhais pes-pelicani, and the application of the specific term will be understood when the shell is viewed from above, for the expanded lip is drawn out into long finger-like lobes that suggest the foot of a bird. This is a very solid shell, sometimes reaching a length of two inches; and the animal inhabiting it is carnivorous.
Fig. 176.—Aporrhais pes-pelicani, showing both shell and animal
We have yet some turreted shells to deal with, belonging to the family Pyramidellidæ, but they need not be confused with the preceding groups if carefully examined. In the first place, the aperture of the shell is very small; and the operculum, instead of being spiral, as in the turreted shells before mentioned, is imbricated or made up of parallel layers denoting that the growth took place on one side only. Another distinguishing feature is seen in the nucleus—that small portion of the spire that was developed within the egg—which is sinistral or left-handed. In addition to this, the animal has broad, ear-like tentacles, a retractile proboscis, and a lingual ribbon without teeth.
The British species of this family belong principally to the genera Odostomia, characterised by a tooth-like fold of the columella; Eulima, containing small, white, polished shells with numerous level whorls; and Aclis, with little polished shells not unlike Turritella.
Fig. 177.—1. Odostomia plicata. 2. Eulima polita. 3. Aclis supranitida
The last family of the Holostomata is the Naticidæ, the shells of which are almost globular, with only a few whorls, and a small, blunt spire. The mouth is semilunar in form, and the lip sharp. The proboscis of the animal is long and retractile, and the foot large; but perhaps the most characteristic feature is the presence of large mantle lobes which hide some of the shell when the animal is crawling. In Natica (fig. 155), the typical genus, the shells are somewhat thick and smooth, with a large umbilicus. As the animal crawls a large fold of the mantle is reflected back over the head, completely covering it, and apparently obstructing its view; but this is not the case, for the creature has no eyes. Natica is very abundant on some sandy beaches, where it devours small bivalves and other animals; and it is frequently washed up alive by the waves. Its shell is also a favourite one with hermit crabs. Its eggs, all connected together in a spiral band, may often be seen stranded on sandy coasts. Several species of Natica are found on our shores. An allied mollusc—Velutina lævigata, so called on account of the velvety epidermis that clothes the shell, completely surrounds the shell by its mantle folds when creeping.
The Siphonostomata form a much smaller section than the last, and its members are distinguished mainly by the presence of a true siphon, formed by the prolongation of the mantle margin, and serving to convey water into the gill chamber. In all these the shell is spiral, usually without an umbilical opening, and the margin of the mouth is prolonged into a canal or distinctly notched. The operculum is horny, and lamellar or imbricated. The animal has a retractile proboscis, and the eyes or eye-pedicels are joined to the tentacles. All the species of this division are marine.
Fig. 178.—Cypræa (Trivia) europæa
We will first take the family Cypræidæ, which contains the familiar Cowries, these forming the lowest group of the division. An examination of the shells may at first seem rather puzzling, for the spire is concealed, and the whole is convoluted in such a manner as to make the mouth long and narrow, with a channel at either end. The outer lip is also thickened and bent inward, and there is no operculum.
The animal itself is particularly interesting, for, as it creeps along on its broad foot, abruptly shortened in the front, the mantle lobes bend over the top, meeting along the middle line, where they are usually fringed with little tentacle-like processes; and, as a result, the whole shell is beautifully enamelled on the outer surface. In all the Cowries the central teeth are single, and the laterals are arranged either in twos or threes.
Perhaps the commonest representative of this family is the pretty little Cypræa (Trivia) europæa (Plate V.), the shells of which are sometimes washed up in large numbers on sandy beaches. The animal lives mainly below low-water level, but it may often be found in the larger rock pools, creeping rapidly over the tangles, and may be easily secured with the aid of a net.
In the same family we have the little Erato (Marginella) lævis, the white shell of which is minutely furrowed along the lips; and also Ovulum patulum (Calpurna patula), so called on account of its fancied resemblance to a poached egg.
We have also several species of Cone shells (family Conidæ) on our coasts, readily recognised by their form, which is a cone, with a long, narrow aperture, partially closed by a minute operculum. As in the last family, the foot is abruptly shortened in front. The head is very prominent, with eyes situated on the tentacles. There are two gills, and the teeth are arranged in pairs.
Fig. 179.—1. Ovulum patulum. 2. Erato lævis
Fig. 180.—Mangelia septangularis and Mangelia turricula
The Conidæ are principally inhabitants of tropical seas, where some very large species exist. Two of the British representatives, both common shells, are shown in fig. 180.
Our next family (Buccinidæ) is so well distributed on our coasts, that it would be difficult, we imagine, to find a spot quite free from its familiar forms. It contains all those creatures commonly known as Whelks, Dog Whelks, and Dog Winkles, ranging from deep water almost to high-water mark.
In all these the shell is notched in front, or the canal is turned abruptly upward. The foot of the animal is broad, the eyes are situated either on the tentacles or at their bases, and there are two gill plumes.
All the species are carnivorous, and some are said to be very destructive to mussels and young oysters.
The Common Whelk (Buccinum undatum, [Plate V.]) lives in deep water, whence it is dredged up largely for the market. Its clusters of egg cases are washed up in large numbers on the beach, where they form one of the commonest materials among the refuse at high-water mark. It is not uncommon, also, especially after storms, to find the unhatched eggs stranded by the waves, and these are so transparent that the embryos, several in each capsule, may be seen within. The hole through which the young escape may also be seen on the inner side.
Fig. 181.—1. Purpura lapillus. 2. Egg Cases of Purpura. 3. Nassa reticulata
The Dog Periwinkle (Purpura lapillus) abounds on all our coasts and is remarkable for the production of a dull crimson or purple fluid that may be obtained from it by pressing on the operculum. This fluid turns to a brighter colour on exposure to air, and is said to have been used largely in former times as a dye. It will be seen from our figure that the spire of this shell is shorter in proportion than that of Buccinum; but both are alike in that the operculum is made up of layers with a nucleus on the external edge.
The other species figured is Nassa reticulata, popularly known as the Dog Whelk, and characterised by a tooth-like projection of the inner lip close to the anterior canal. It is very common near low-water mark, where it may be seen crawling over the rocks on its broad foot, from which project two hornlike appendages in front and two narrow tails behind.
Fig. 182.—Murex erinaceus
From the last family of the gasteropods (the Muricidæ) we select two common species—Murex erinaceus and Fusus antiquus (Plate V.). In both these the anterior canal of the shell is straight and the posterior wanting. The eyes are on the tentacles, and there are two plumed gills. Both are carnivorous species, feeding on other molluscs; and the former is said to bore through the shells of its prey with the prominent beak of its shell.
Murex may be readily distinguished by the prominent longitudinal ridges of the thick shell, its rounded aperture, and by the partly closed canal running through the beak. It is known to fishermen as the Sting Winkle; the other species is called the Red Whelk in some parts, and in Scotland is known as the Buckie. Like the common whelk, it is dredged largely for the market, and is said to be far more esteemed than the former, from which it may be distinguished by the fusiform shape of the shell and the long straight canal.
We now pass to the last and highest class of the mollusca, called the Cephalopoda because they have a number of arms attached to the head, round the mouth. Unlike the majority of molluscs they are bilaterally symmetrical: and are much more highly organised, in some respects even making an approach to the vertebrates. Thus they generally have an internal hard structure, either horny or calcareous in structure, representing the vertebral column, and the circulatory system consists of arteries and veins, connected by minute capillaries. The corpuscles of the blood are also similar in form to those of the vertebrates. Externally they are all naked, with the exception of the nautilus and argonaut of the warmer seas.
The arms, so characteristic of the class, are eight or ten in number, long and muscular, and provided with numerous suckers by which the animal can cling with remarkable tenacity. These suckers are situated on the inner surface of the arms, and the disc of each one displays a series of muscular fibres, all converging from the circumference towards the centre, which is occupied by a softer structure that works inwards and outwards like the piston of a pump. Thus the suckers form a system of exhausting air-pumps by which a vacuum can be produced, and the tenacity of the grip, maintained by atmospheric pressure, is so great that the arms, strong as they are, may be torn asunder by attempting to pull them from their hold; and yet the animal can release its grip with the greatest of ease by simply releasing the pistons of its pumps.
The cephalopods are further distinguished by their very large, glaring eyes, situated on the sides of the well-formed head, and by powerful jaws that work in a vertical plane, like those of the vertebrates, but somewhat resembling the beaks of certain birds. The tongue is also very large and fleshy, and in part armed with numerous hooked spines or teeth.
The class is usually divided into two orders, one characterised by the possession of two gills, and the other of four; but the British species belong to the former, known technically as the Dibranchiata. This order is subdivided into two sections according to the number of arms; and the divisions are called the Octopoda and Decapoda respectively.
Fig. 183.—Octopus
The former section includes the Octopods, of which some species inhabit our seas. They all have eight arms, of unequal size, with the suckers arranged in two rows, and their round or oval bodies seldom have any fins, locomotion being effected by means of the arms, and by the sudden expulsion of water from the siphon. The shell is rudimentary, being represented merely by two short ‘styles’ within the mantle. The species vary considerably in size, some being only about an inch long when fully grown, while others measure two feet or more, and are looked upon as formidable creatures by man. Sometimes they are washed up on our beaches, but the best way to make their acquaintance is to examine the contents of the fishermen’s drag nets as they are hauled on the beach.
In the same manner we may secure various species of the Decapods or Ten-footed Cephalopods, which comprise the Calamaries, Squids, and Cuttlefishes. These, too, properly speaking, have but eight arms, the other two appendages being really tentacles, which are usually longer than the arms, and more or less retractile; they are also expanded at the ends. The decapods are also to be distinguished from the octopods by their elongated bodies, and a flattened, fin-like appendage on either side. Their eyes, also, are capable of being rotated within the orbits, while those of the octopods are fixed; and the shell consists of one or more horny ‘pens,’ or of a calcareous ‘bone,’ contained in a cavity so loosely that it drops out of its place when the cavity is opened.
Fig. 184.—Loligo vulgaris and its Pen
Fig. 185.—Sepiola atlantica
The Common Calamary (Loligo vulgaris) may be recognised by the accompanying illustration, from which it will be observed that the body tapers behind, bearing two rhomboidal fins in the rear. The suckers are arranged in two rows on the arms, but in fours on the expanded tips of the tentacles. The animal is a good swimmer, and sometimes crawls, head downwards, on the disc surrounding the mouth, pulling itself along by means of its arms. Its shell is a horny pen, lanceolate in form, but it divides as the age of the animal advances, so that two or more may be found in the same specimen.
Belonging to the same family we have the Common Squid (Sepiola atlantica), also a very abundant species. Here the body is shorter and purse-like, and the fins are dorsal and rounded. It seldom exceeds four or five inches in length, and, like the Calamary, is used largely as a bait by fishermen.
Another family—the Sepiadæ—contains the Cuttlefish (Sepia officinalis), the ‘bone’ of which is such a common object on the beach. This latter is a broad, curved plate of carbonate of lime, made up of a number of regular layers, and having a cavity hollowed out at the posterior end. It is exceedingly light and porous in structure, and at one time was used largely as an antacid as well as a dentifrice. It is also proportionately large, being both as long and as broad as the body of the animal.
Fig. 186.—Sepia officinalis and its ‘Bone’
Cuttlefishes live principally in the shallow water close to shore, where they swim backwards by the sudden propulsion of water from their siphons; and their eggs, which look like clusters of black grapes, are frequently thrown up on the beach, generally attached to the stems and fronds of sea weeds.
As a rule the cephalopods swim slowly by the aid of their fins or by a rhythmic contraction by which water is expelled from their siphons, but when in danger the muscular contraction is so violent that they dart through the water with great speed, and even leap into the air to avoid their enemies. But they have another and much more remarkable way of escaping from their foes:—They possess a gland, the duct of which opens into the base of the funnel or siphon, that prepares an inky fluid; and when the animal is disturbed it suddenly ejects this fluid, rendering the surrounding water so cloudy that it is often enabled to retreat unobserved. The ‘ink’ of the Sepia was used for writing in former times, and is still employed in the preparation of the artist’s pigment that bears the same name. Fishermen are well acquainted with this peculiar characteristic of the animal, for they are frequently bespattered with the contents of the ink bag of the Sepia when the creature is included in the contents of their draw-nets, and have learnt to handle it cautiously until the objectionable fluid has been all discharged.
Fig. 187.—Eggs of Sepia
We will conclude this chapter by giving a tabular summary of the classification of the molluscs which will probably be useful to the collector of marine objects.