GENERAL NOTES ON MOLLUSCA.
The Mollusca constitute one of the principal divisions of the Animal Kingdom, and include such animals as the Octopus, Cuttlefish, Snail, Slug, Whelk, Cockle, and Oyster.
Definition.
They may be characterized as soft, cold-blooded animals, without distinctly marked external division into segments (as in Worms); their cerebral ganglia (the centre of the nervous system) lie above the commencement of the gullet, and are connected with the inferior ganglia by nerve-chords. Their heart consists of two or more chambers, and is situated on the dorsal side of the animal; it drives the blood into spaces between the various organs of the body. Only the Cephalopods possess internal cartilages, but all are without a bony internal skeleton; in the majority this is compensated by an external hardened shell which is formed (secreted) by the outer covering of the animal termed the mantle. |The shell.| The shell may consist of two parts (valves), as in the Oyster, or may be single, as in the Whelk and Limpet, or composed of a series of plates, as in the “Coat-of-mail” shells or Chitons: when well developed it is hardened by a rich deposit of carbonate of lime; but it may be gelatinous, as in Cymbulia, or altogether absent, as in Octopus; it may cover and protect the body, as in the Oyster, lie within the folds of the mantle, as in the Sea-hares (Aplysiidæ), or it may be quite internal, as in the horny “pen” of the Squid. It may be elongated, as in the Elephant Tooth-shell (Dentalium), cup-shaped, as in the Limpet, or spirally coiled, as in the Snail.
Description of the animal.
The mantle may form a free fold on either side of the body, as in the Bivalves, or it may become largely attached to the body-wall, as in the Snail or the Slug, and so give rise to an air-chamber, which, when its walls are richly supplied with blood, serves as a lung. The ventral surface of Molluscs is produced into the so-called “foot,” which may be very variously modified. The foot may be more or less hatchet-shaped, or curved and capable of serving as a leaping-organ, or sole-shaped and adapted for creeping; its margins may be produced into elongated processes, as the so-called arms of the Octopus, eight in number and provided with suckers, or of the Nautilus, where the arms are much more numerous, but shorter and without suckers. In the Cephalopods, also, another part of the foot may fold over from either side and form a median funnel, through which the water of respiration is driven outwards, causing the animal to move in the opposite direction—this part of the foot having, therefore, still the function of an organ of locomotion. By means of their muscular foot the Solenidæ, or Razor-shells, burrow in the sand, the Pond-Snails (Limnæidæ) crawl on aquatic plants and swim reversed on the surface of the water, the Limpet clings to the rock, and the Cockles and Trigonias take surprising leaps.
The operculum.
Upon the upper surface of the foot, in many Gastropods, a flat hard structure termed the operculum is situated, which, when the animal is retracted, partly or entirely closes the aperture of the shell. In some cases, as in the Turbos, it is very strong and of a stony nature, but in most instances it is horny. It is differently constructed in distinct families: it may be annular and multispiral, annular and paucispiral, subannular and ovate, or subannular and unguiculate. In the Nerites it is shelly, somewhat semicircular, closes the aperture of the shell, and is furnished with a stout projection on the straight edge, fitting like a hinge under the inner lip of the shell. A series of opercula is exhibited in side table-case C.
The breathing-organs.
Thread-like processes on either side of the body, the so-called gill-filaments, often unite with those in front of and behind them, and so give rise to plates; these, when well developed, are best seen in the division to which the Oyster and the Mussel belong, and which, therefore, has been called the division of the plate-gilled Molluscs, or Lamellibranchia. Where the body is coiled or twisted on itself, as so often happens, the gills of one side may be altogether lost. Sometimes, as in Phyllirhoë, when the body is small and its wall thin, the gills (ctenidia) disappear altogether, and there is no special breathing-organ; in others the loss of the gill is compensated by the formation by the mantle of a lung; this is most often seen in the forms that live on land.
But these so-called gills may have other functions: in the Lamellibranchs, where there is no head and no special means by which the creature can obtain food, the delicate waving filaments or cilia with which they are covered cause currents in the surrounding water, by means of which minute organisms are brought to the mouth.
The radula.
All Molluscs, except the Lamellibranchs, have a very remarkable structure developed in the floor of their mouth-cavities; on a basis of cartilage, which may be moved backwards and forwards by muscles, there is developed a horny plate, which may be of considerable length, and which has its upper surface covered with a number of more or less fine, flattened, or spiny outgrowths, which are known as teeth. This is the odontophore, tongue, radula, or lingual ribbon (see fig. 3).[[1]]
The eyes.
Eyes may be absent, as in nearly all the headless Lamellibranchs; but in other Molluscs they are generally present, and may be more or less well developed. An instructive series of stages is exhibited by the Cephalopoda. In Nautilus the eye remains an open pit; in Ommatostrephes two chambers appear, the anterior of which is bounded posteriorly by the lens, and is open to the exterior, so that sea-water enters it; in Sepia, finally, the anterior chamber becomes closed in front. We may observe that the eyes of all Cephalopods are at first pit-like, or pass through a stage which is permanent in Nautilus, one of the geologically oldest types.
Cephalic eyes have recently been noticed in Mytilus and Avicula.
Eyes of a more complicated structure, which are modified tentacles, are sometimes found on the edges of the mantle in Lamellibranchs (e.g. Pecten); these eyes resemble those of Vertebrates, and differ from those of most invertebrate animals in having the fibres of the optic nerve entering the distal and not the proximal ends of the retinal cells. Eyes of a similar construction are to be found on the back of the shellless Oncidium, and may be about one hundred in number.
Eyes of a remarkable character on the shells of some of the Chitons appear to be modified from tactile organs, and are innervated like the ordinary molluscan eye; they sometimes occur in enormous numbers, more than ten thousand being present on one animal (see wax model, Case 2).
Organ of hearing.
In Cephalopods the ear, like the eye, is known to make its first appearance in the form of an open pit, the mouth of which gradually closes up, leaving only a narrow slit in communication with the exterior. It is probable that in many forms the so-called ear is an organ by means of which the mollusc becomes acquainted with changes in the surface over which it is passing; it is often found deeply imbedded in the substance of the foot, where it forms a closed vesicle.
Sense of smell.
There is no doubt that the carnivorous Gastropoda are gifted with a sense of smell, and throughout the series we observe patches of modified cells of the body-wall (the osphradium) which serve either as olfactory organs or as an apparatus for testing the nature of the water of respiration.
The sexes and reproduction.
The sexes are distinct in the most highly organized Mollusca, but are united in the same individual in some of the lower forms, such as Land-Snails, the Opisthobranchia (including the Bubble-Shells, Sea-Slugs, &c.), and in some Bivalves. The reproduction of Mollusca is in all cases effected by means of eggs. In some instances the young are actually hatched within the oviduct of the parent, as in the Freshwater Snails (Vivipara); and apparently in most Bivalves the eggs are also retained within the valves until hatched.
The ova of many molluscs are deposited in masses enclosed in capsules. Some of them are very wonderful and complicated structures. Those of the Cuttles and their allies are clustered like grapes, each capsule containing but a single embryo; but in the Calamaries or Squids they form a radiating mass of elongated sacks, each containing from thirty to two hundred eggs, and it has been estimated that one of the spawn-clusters of the Common Squid (Loligo vulgaris) contains as many as 40,000 ova. Everybody knows the spawn-cases of the Common Whelk, found so abundantly on the sea-beach, consisting of a large number of yellowish capsules, heaped one upon another and forming an irregularly rounded mass. As many as five or six hundred capsules may be piled together in a single heap, each capsule containing several hundred eggs, of which perhaps only thirty or forty are hatched.
In other genera, as Tethys, Doris, Eolis, &c., the eggs are contained in a spirally rolled ribbon or strap-like structure; and some of the Naticæ build a somewhat similar capsule, composed of the eggs cemented together by sand and a gelatinous material, the whole forming two-thirds of a circle narrowed at the upper part.
Terrestrial Molluscs deposit, in comparison with their marine relations, but very few eggs. They are sometimes covered by a thin soft skin, but in certain groups, such as the large South-American Strophochili and the African Achatinæ, which include the largest of known land-molluscs, they are protected by a hardened calcareous shell, in some instances fully an inch in diameter. The freshwater forms (Limnæa and Physa) deposit from thirty to a hundred eggs enveloped in a gelatinous mass.
The number of eggs produced by some Bivalves is enormous. The Common Oyster is said to produce a million or more, and the American variety ten, or even sixty, times as many. Some of the River-Mussels are also very prolific, as many as two millions being sometimes the product of a single individual. A small series of the eggs of Land-Snails and of the egg-capsules of some marine Gastropods is exhibited in side table-case C at the side of the Gallery.
The ova of Mollusca may be gradually developed into the form of the parent, or there may be a free-swimming larva, which has a circlet of cilia near the anterior pole of its body (so-called “Veliger” larvæ), or there may be special larvæ, as in the case of the Freshwater Mussel, the “Glochidium,” as it is called, which has a toothed bivalve shell by which it can fix itself to fishes.
Duration of life.
The limits of age of molluscs has been definitely ascertained in a few instances only. Most Land-Snails probably live about two years, although in confinement some have been kept alive for a much longer time. Some of the marine forms live for a considerable period, the Common Oyster not attaining full growth until about five years old, after which it may continue to live for many years. The Giant Clam, a specimen of which is placed in the upright cases near the entrance to the Gallery, must, one would think, have a very long existence, judging from the size and thickness of the shell. |Hibernation and torpidity.| All terrestrial molluscs hibernate in cold climates, hiding themselves away in the ground between roots and similar sheltered places. In tropical countries some assume a state of torpidity (æstivate) during the hottest and driest season of the year, closing up the aperture of their shells with a temporary lid or door (epiphragm), in order to resist the dryness of the atmosphere. Some of these “summer-sleepers” are endowed with a remarkable tenacity of life. An Australian Pond-Mussel has been known to live a year after being removed from the water; several Land-Snails have revived after a captivity of from two to five years, without any food whatever. One of the most remarkable instances of this kind occurred in the British Museum. A specimen of Helix desertorum, a common Desert-Snail from Egypt, was fixed to a tablet in March 1846, and in the same month of the year 1850 it was discovered to be alive. It must have come out of its shell in the interval, and finding it was unable to crawl away, had again retired within it, closing the aperture with a new epiphragm, but leaving traces of slime upon the tablet, which led to its immersion in water and subsequent revival, having passed a period of four years in a dry museum without the smallest particle of food. The actual specimen is here figured, Fig. 1.
Fig. 1.[[2]]
Helix desertorum.
(See black table-case 1.)
Economic uses.
The economic uses of molluscs to man are manifold, and will be mentioned in the course of the description of the several families; but here may be the place to direct the attention of visitors to side table-cases B and D at the side of the room, containing some specimens of articles manufactured from shells, such as cameos, flowers, bracelets, brooches, &c.
Geological history.
Mollusca made their appearance on the globe at a very early epoch in the history of the development of animal life, a large number of fossil forms, such as Nautilus, Lituites, Orthoceras, &c., being found in the oldest Palæozoic formations. Probably all these belonged to the Tetrabranchia, of which one descendant only, the Pearly Nautilus, has survived to our period. Some Gastropods and Bivalves coexisted with those ancient Tetrabranchs; but these types abounded more in the later geological epochs, many Tertiary forms being undistinguishable from species which now exist.
General distribution.
The greater number of Mollusca are inhabitants of the sea, some passing their whole life at the surface hundreds or thousands of miles away from land; others at the bottom of the ocean at all depths, some having been dredged at five miles from the surface. Many are found in much shallower water, and a large number between tide-marks. Rivers and lakes furnish an immense variety of forms, and vast numbers live on land in all situations—on mountains, in valleys, forests, and deserts.
Molluscs are either animal or vegetable-feeders, the former preying principally upon other members of their own class.
Systemic arrangement.
The following Table shows the systematic arrangement of the Mollusca adopted in the Shell Gallery:—
Class I.—Amphineura.
Order 1.— Polyplacophora. Chitons. „ 2.— Aplacophora. Neomenia, Chætoderma, etc.
Class II.—Gastropoda.
(Section Streptoneura.)
Order 1.— Scutibranchia. Nerites, Top-shells, Ear-shells, Limpets. „ 2.— Pectinibranchia. Rock-snails, Whelks, Olive-shells, Harp-shells, Cones, Strombs or Wing-shells, Periwinkles, Carrier-shells, etc.
(Section Euthyneura.)
Order 1.— Opisthobranchia. Bubble-shells, Sea-hares, Umbrella-shells. „ 2.— Pulmonata. Land and freshwater Snails, False Limpets.
Class III.—Scaphopoda. Tooth-shells (Dentalium).
Class IV.—Lamellibranchia. Bivalved Molluscs.
Order 1.— Protobranchia. Nucula, etc. „ 2.— Filibranchia. Anomia, Common Mussel, Ark-shells, etc. „ 3.— Pseudolamellibranchia. Pearl-Oyster, Hammer Oyster, Wing-shells, etc. „ 4.— Eulamellibranchia. Freshwater Mussels, Cockles, Razor-shells, Ship-worms, etc. „ 5.— Septibranchia. Poromya, etc.
Class V.—Cephalopoda.
Order 1.— Tetrabranchia. Pearly Nautilus. „ 2.— Dibranchia. Octopus, or Poulp, Argonaut, Squids, and Cuttle fishes.