ON
MOLECULAR AND MICROSCOPIC SCIENCE
VOLUME THE SECOND

LONDON: PRINTED BY

SPOTTISWOODE AND CO., NEW-STREET SQUARE

AND PARLIAMENT STREET

Fig. 118, p. 107.

GALEOLARIA LUTEA.

[Frontispiece to Vol. II.

ON

MOLECULAR

AND

MICROSCOPIC SCIENCE

BY MARY SOMERVILLE

AUTHOR OF ‘THE MECHANISM OF THE HEAVENS’ ‘PHYSICAL GEOGRAPHY’

‘CONNECTION OF THE PHYSICAL SCIENCES’ ETC.

Deus magnus in magnis, maximus in minimis—St. Augustine

In Two Volumes—Vol. II.

WITH ILLUSTRATIONS

LONDON

JOHN MURRAY, ALBEMARLE STREET

1869

The right of translation is reserved

CONTENTS
OF
THE SECOND VOLUME.

PART III.

ANIMAL ORGANISMS.

ILLUSTRATIONS
TO
THE SECOND VOLUME.

[A]. From Dr. Ernst Haeckel’s ‘Radiolarien.’

[B]. From Voght’s ‘Syphonophores de la Mer de Nice’.

MOLECULAR AND MICROSCOPIC SCIENCE.

PART III.
ANIMAL ORGANISMS.

SECTION I.
FUNCTIONS OF THE ANIMAL FRAME.

Although animal life is only known to us as a manifestation of divine power not to be explained, yet the various phases of life, growth, and structure in animals, from the microscopic Monad to Man, are legitimate subjects of physical inquiry, being totally independent of those high moral and religious sentiments which are peculiar to Man alone.

The same simple elements chemically combined in definite but different proportions form the base of animal as well as of vegetable life. But besides the elementary gases and carbon, many substances, simple and compound, are found in the animal frame; the phosphate and carbonate of lime, iron which colours the blood, and common salt which, with the exception of water, is the only article of food we use in a mineral state. Animals derive their nourishment, both directly and indirectly, from vegetables. Their incapacity to change inert into living matter is one of the most characteristic distinctions between the animal and vegetable kingdoms.

Protoplasm was shown to be rudimentary formative vegetable matter: so Sarcode, or rudimentary flesh, forms the whole or part of every animal structure. It is a semi-fluid substance, consisting of an albuminous base, mixed with particles of oil in a state of very fine division. It is tenacious, extensile, contractile, and diaphanous, reflecting light more than water, but less than oil. It is rendered perfectly transparent by citric acid, and is dyed brown by iodine. This substance, in a homogeneous state, constitutes the whole frame of the lowest grade of animal life; but when gradually differentiated into cell-wall and cell-contents, it becomes the origin of animal structure from that which has little more than mere existence to man himself; in fact, cellular origin and cellular structure prevail throughout every class of animal life. Unicellular plants and animals live for themselves independently and alone; but the cells which form part of the higher and compound individuals of both kingdoms, may be said to have two lives, one peculiarly their own, and another depending on that of the organized beings of which they form a part.

Flesh or muscle, which is organized sarcode, consists of two parts, namely, bundles of muscular fibre imbedded in areolar tissue. Nervous matter also consists of two parts, differing much in appearance and structure, the one being cellular, the other fibrous. The vital activity of the nerves far surpasses that of every other tissue; but there is an inherent irritability in muscular fibre altogether independent of nervous action: both the nervous and muscular tissues are subject to decay and waste.

The blood, which is the ultimate result of the assimilation of the food and respiration, conveys nourishment to all the tissues during its circulation; for with every breath, with every effort, muscular or mental, with every motion, voluntary or involuntary, at every instant of life, asleep or awake, part of the muscular and nervous substances becomes dead, separates from the living part, is returned to the circulation, combines with the oxygen of the blood, and is removed from the system, the waste being ordinarily in exact proportion to the exertion, mental and physical. Hence food, assimilated into blood, is necessary to supply nourishment to the muscles, and to restore strength to the nervous system, on which all our vital motions depend; for, by the nerves, volition acts upon living matter. Waste and repair is a law of nature, but when nature begins to decay, the waste exceeds the supply.

However, something more than food is necessary, for the oxygen in the blood would soon be exhausted were it not constantly restored by inspiration of atmospheric air. The perpetual combination of the oxygen of the air with the carbon of the blood derived from the food is a real combustion, and the cause of animal heat; but if the carbonic acid gas produced by that chemical union were not continually given out by the respiratory organs, it would become injurious to the animal system. Thus respiration and the circulation of the blood are mutually dependent; the activity of the one is exactly proportional to that of the other: both are increased by exercise and nervous excitement.

External heat is no less essential to animals than to vegetables; the development of a germ or egg is as dependent on heat as that of a seed. The amount of heat generated by respiration and that carried off by the air is a more or less constant quantity; hence, in hot countries, rice and other vegetable diet is sufficient, but as the cold increases with the latitude, more and more animal food or hydrocarbon is requisite for the production of heat.

The waste of the tissues, and the aëration of the vital juices, that is, the exchange of the respiratory gases, are common to all animals. The heart, upon whose expansions and contractions the circulation of the blood depends, is represented in the lower animals by propelling organs of a variety of forms; and the organs of respiration differ exceedingly, according to the medium in which the animals live. Water, both fresh and salt, though a suffocating element to land animals, contains a great deal of air, not only in the state of gas, but also in solution, the quantity in solution being directly as the pressure; so that animals living in the deepest recesses of the ocean breathe as freely as those that live on land, but with respiratory organs of a very different structure. In the lowest classes, which have no respiratory organs at all, the gases are exchanged through their thin delicate skins.

The mechanical forces act within the living being according to the same laws as they do in the external world: the chemical powers too, which are the cause of digestion, heat, and respiration, follow the same laws of definite and quantitative proportion as they do in inert matter; but neither the mechanical forces, nor the physical powers, could create a germ; nor could they even awaken its dormant state to living energy, unless a vital power existed in it, the origin of which is beyond the reach of man.

Animals are endowed with nerve-force, in addition to mechanical force and the physical powers which are common to them and vegetables; a force which constitutes their prime distinction, which is superior to all the other powers from its immediate connection with mind, and which becomes more evident, and more evidently under the control of the animal, in proportion as the animal approaches the higher grades of life, and only attains its perfect development in the human race.

The bones of man and the higher animals are clothed with a system of muscles, so attached that the head, eyes, limbs, &c., can be moved in various directions. In each of these muscles the fibres of two sets of nerves ramify, namely, the sensory and the motor nerves.

The sensory nerves convey external impressions to the brain, and by them alone the mind is rendered conscious of external objects. The impressions made by light and sound upon the eye and the ear, or by mechanical touch on the body, are conveyed by the sensory nerves to the brain, where they are perceived, though the impressions take place at a distance from it. Conversely, the mind or will acts through the brain on the motor nerves, which by alternately contracting, relaxing, and directing the muscles, produces muscular motion. Thus the motor nerves convey the emotions of the mind to the external world, and the sensory nerves convey the impressions made by the external world to the mind. By these admirable discoveries, Sir Charles Bell has proved that ‘we are placed between two worlds, the invisible and the material;’ our nervous system is the bond of connection. The connection, however, between the mind and the brain is unknown: it has never been explained, and is probably inexplicable; yet it is evident that the mind or will, though immaterial, manifests itself by acting on matter; that is, as a power which stimulates the nerves, the nerve-force acting on the muscles. Mental excitement calls forth the most powerful muscular strength, and an iron will can resist the greatest nervous excitement. The nervous and muscular forces are perpetually called into action, because, for distinct perception, the muscles require to be adjusted. Mind is passive as well as active: we may see an object without perceiving it, and we may hear a sound without attending to it. We must look in order to see, listen in order to hear, and handle in order to feel; that is, we must adjust the muscular apparatus of all our senses, of our eyes, ears, &c., if we would have a distinct perception of external exciting objects: and that is accomplished by the power of mind acting upon matter.

Dr. Carpenter has shown that it is by a series of forces acting upon matter that man conveys his ideas to man, the sonorous undulations of the atmosphere being the medium between the two. On one side the will, or power of mind, acts upon the nerves, nerve-force acts upon the muscles of speech, and these muscles, while in the act of speaking, produce sonorous undulations in the atmosphere. On the other side, these undulations are communicated by the mechanism of the ear to the auditory nerves, exciting nerve-force, and nerve-force acts upon the mind of the hearer. ‘Thus the consciousness of the speaker acts upon the consciousness of the hearer by a well-connected series of powers.’

Nerve-force generates, directly or indirectly, light, heat, chemical power, and electricity. When the optic nerve is pressed in the dark, a luminous ring is seen round the eye, and a blow on the face excites a flash of light. Nervous excitement, by accelerating respiration, increases the chemical combination of the oxygen of the air with the carbon of the blood, and thus produces animal heat. But the development of electricity by nervous and muscular force, is one of the most unexpected and singular results of physiological research.

MM. Matteucci and Du Bois Reymond have proved that the intensity of the nervous and muscular forces is at a maximum when the muscles are contracted; and that if each arm of a man be put in contact with a wire of a galvanometer so as to form an electric circuit, an instantaneous deviation of the needle will take place, now in one direction and now in the other, according as he contracts his right arm or his left. The electricity thus evolved, when conveyed to the needle through several miles’ length of coiled insulated wire, will cause a deflection amounting to sixty or seventy degrees, according to the strength of the man—that is, according to his muscular and nervous force; the amount of the electricity being exactly in proportion to the amount of muscular force.

It appears that the electric currents in the nerves are eight or ten times stronger than those in the muscles. M. Helmholtz found that the time required to contract a muscle, together with the time required to relax it again, is not more than the third of a second, and is a constant quantity, for the compensation of energy prevails also in organic nature. He also found that the motion or velocity of the electric current in a man is at the rate of 200 feet in a second. The electric equivalent, as determined by M. Helmholtz, is equal to the electricity produced in a voltaic battery by the seven millionth part of a milligramme of zinc consumed in the ten-thousandth part of a second, a milligramme being the 0·015432 part of a grain.

The contraction and muscular action or mechanical labour produced by the passage of an electric current through a nerve is 27,000 times greater than the mechanical labour which results from the heat disengaged by the oxidation of that small quantity of zinc requisite to generate the electricity; that is to say, the mechanical labour really produced by the contraction of the muscles is enormously greater than the labour corresponding to the zinc oxidized. In fact, the electric excitement of a nerve is analogous to an incandescent particle or electric spark that sets fire to a great mass of gunpowder. This result, and the association between the greatest activity of respiration and the intensity of the muscular energy, led M. Matteucci to suspect that a chemical action must take place in the interior of a muscle during its contraction; and he found by experiment that there actually is what he calls a muscular respiration, namely, that the muscles themselves absorb oxygen, and give out carbonic acid gas and nitrogen when contracted. This kind of respiration is more or less common to all animals; if impeded, the blood is imperfectly oxygenized, and loss of animal heat is the consequence. The heat that is perpetually escaping from animals is replaced, by the combustion of the carbon of the tissues or of the food with the oxygen inhaled by the lungs and the skin.

In the highest class of animal life the brain is at once the seat of intelligence and sensibility, and the origin of the nervous system. In the lower animals intelligence and sensibility decrease exactly in proportion to the deviation of their nervous system from this high standard. The forms of the nervous system are more and more degraded as the animals sink in the scale of being, till at last creatures are found in which nerves have only been discovered with the microscope; others apparently have none, consequently they have little or no sensibility.

The brain and the spinal cord enclosed in the vertebræ of the backbone form a nervous system, which in the vertebrated creation is equal to all the contingencies and powers of these animated beings, but is beyond all comparison most perfect in the human race. The brain alone is the seat of consciousness, for the spinal cord, though intimately connected with it, and of a similar ‘mysterious albuminous electric pulp,’ appears to have no relation to the faculties of perception and thought, yet it is essential to the continuance of life. It is a distinct nervous centre which generates muscular energy in man and animals corresponding to external impressions, but without sensation, and is entirely independent of the will; the vegetative functions of respiration, the contractions of the heart, circulation of the blood, and digestion, are carried on under every circumstance, even during sleep. The reason of their being independent of sensation and the will is, that the nerves in the organs performing these functions never reach the brain, which is the seat of intelligence and sensation, but they form what is called the reflex system; for any impressions made upon them are carried to the upper part of the spinal cord alone, and are reflected back again to the muscles of the heart, lungs, &c., which, by their contractions, produce these involuntary motions. For instance, the flow of blood into the cavities of the heart while dilating, acts upon the nerves, and these excite a rhythmical movement in the muscular fibres of the heart. For there is a vital contractility in muscular tissue which is one of the most universal attributes of living beings, and is probably the sole cause of motion in the lowest grades of life, and the movements produced by it in the higher grades are in all cases the most directly connected with the vegetative functions. The involuntary reflex system of nerves constitutes the chief locomotive power in a number of the lower animals; but it forms a continually decreasing portion of the whole nervous system in proportion as animals rise in the scale of life, till in man its very existence has been overlooked. If the spinal cord were destroyed, instant death would be the consequence; whereas infants born without brain have sucked and lived for a day or two.

There are numerous actions, especially among the lower animals, as little under the influence of the will or intelligence as the reflex nerves, which nevertheless depend upon sensation for their excitement. The sensation may call the muscular apparatus into action without any exertion of reason or will, in such a manner as to produce actions as directly and obviously adapted to the well-being of the individual as the reflex system. For example, a grain of dust irritates the nostrils, and involuntarily excites the complicated muscular movements concerned in the act of sneezing. This class of actions, which is called sensori-motor, or consensual, includes most of the purely instinctive motions of the lower animals, which, being prompted by sensations, cannot be assigned to the reflex group.

Purely emotional movements are nearly allied to the preceding. Sensation excites a mental feeling, or impulse, which reacts upon the muscular system without calling either the will or the instinct into exercise. These emotional movements are often performed in opposition to the strongest efforts of the will, as when a sense of something ridiculous may excite irresistible laughter at an improper time. It is probable that the strong emotions exhibited by many of the lower animals, which have been ascribed to instinct, are referable to this group.[^[1]]

The movements of such animals as have no nerves are merely owing to the vital contractility of muscular fibre.

In the highest province of animal life, which includes the mammalia, birds, reptiles, and fishes, the general structure of the nervous system consists of a double lobed brain, from whence a spinal cord proceeds, protected by articulated bones, which extend along the back of the animals, and from thence nerve-fibres extend to every part of the body. But in order to suit a great variety of forms, this system undergoes many modifications. In all the lower grades of life that have nerves, the system chiefly consists of small globular masses, or nuclei, of nervous matter, technically called ganglia, which are centres of nervous energy, each of which is endowed with its own peculiar properties; the nervous cords and filaments proceeding from them are merely organs of transmission. The arrangement of these centres of nerve-force is symmetrical, or unsymmetrical, according to the form of the animal.

In the lower portion of Articulated animals, such as insects, crustacea, annelids, worms, &c. &c., there is a double cord extending along the ventral side of the animal, united at equal intervals by double nerve-centres, or ganglia. These two cords diverge towards the upper end, surround the gullet, and unite again above that tube to form a distinct bilobed principal nerve-centre or brain. A third form of the nervous system is only a ring round the gullet; the points in it from whence the nerves radiate are swollen nerve-centres, or ganglia. Those on the sides and upper parts of the ring represent the brain, and supply the eyes, mouth, &c., with nerves: other centres, connected with the lower side of the ring, send nerves to the locomotive organs, viscera, and respiratory organs. In animals of a still lower grade there are single nuclei irregularly scattered, but in every case they are centres of energy from whence filaments are sent to the different parts of the creature. The last and lowest system consists of filamentous nerves, chiefly microscopic.

Intelligence, or the mental principle, in animals differs in degree, though not in kind, from that in the human race. It is higher in proportion as the nervous system, especially the brain, approximates in structure to that of man; but even in many of the lower orders may be traced the dawn of that intelligence which has made man supreme on earth. Every atom in the human frame, as well as in that of other animals, undergoes a periodical change by continual waste and renovation; but the same frame remains: the abode is changed, not the inhabitant. Yet it is generally assumed that the living principle of animals is extinguished when the abode finally crumbles into dust, a tacit acknowledgment of the doctrine of materialism; for it is assuming that the high intelligence, memory, affection, fidelity, and conscience of a dog, or elephant, depend upon a combination of the atoms of matter. To suppose that the vital spark is evanescent, while there is every reason to believe that the atoms of matter are imperishable, is admitting the superiority of matter over mind: an assumption altogether at variance with the result of geological sequence; for Sir Charles Lyell observes, that ‘sensation, instinct, the intelligence of the higher mammalia bordering on reason, and lastly the improvable reason of man himself, presents us with a picture of the ever-increasing dominion of mind over matter.’

The physical structure of a vast number of animals has been investigated from such as are a mere microscopic speck to the highest grade of animal life; but very little is comparatively known of their intelligence and means of communication. We know not by what means a pointer and greyhound make an agreement to hunt together; nor how each dog is not only aware that his companion possesses a property which he has not, but that by their united talents they might accomplish their purpose, which is merely sport, for they never eat the game.[^[2]] The undulations of the air and water are no doubt the means by which most animals communicate; but there is reason to believe that many inhabitants of the earth, air, and water are endowed with senses which we do not possess, and which we are consequently incapable of comprehending.

SECTION II.
PROTOZOA.

The Protozoa are the very lowest forms of animal existence, the beginning and dawn of living things. They first appear as minute shapeless particles of semi-fluid sarcode moving on the surface of the waters. The pseudopodia, or false feet, with which they move, are merely lobes of their own substance which they project and retract. In creatures of a somewhat higher grade the form is definite, the pseudopodia, numerous and filamental, serving for locomotion and catching prey; and from the resemblance they bear to the slender roots of plants are called Rhizopods.[^[3]] The microscopic organisms possessing these means of locomotion and supply, are of incalculable multitudes, and of innumerable forms. Thus the waters, as of old, still ‘bring forth abundantly the moving creature that hath life;’ in them the lowest types of the two great kingdoms have their origin, yet they are diverse in the manifestation of the living principle, that slender but decided line which separates the vegetable from the animal Amœba.

Class I.—Rhizopoda.

The Amœba, which is the simplest of the group, is merely a mass of semi-fluid jelly, ‘changing itself into a greater variety of forms than the fabled Proteus, laying hold of its food without members, swallowing it without a mouth, digesting it without a stomach, appropriating its nutritious material without absorbent vessels or a circulating system, moving from place to place without muscles, feeling (if it has any power to do so) without nerves, multiplying itself without eggs, and not only this, but in many instances forming shelly coverings of a symmetry and complexity not surpassed by those of any testaceous animal.’

Fig. 86. Amœba princeps.

Such is the description given by Dr. Carpenter of the Amœba and its allies. The Amœba princeps, which is the type of the naked group, [fig. 86], is merely a shapeless mass of semi-fluid sarcode, coated by a soft, pellucid and highly contractile film, called diaphane by Mr. W. J. Carter, and in many forms of Amœba the whole is inclosed in a transparent covering. It is in the interior semi-fluid sarcode alone, that the coloured and granular particles are diffused, on which the hue and opacity of the body depend, for the ectosarc or external coat is transparent as glass. These creatures, which vary in size from the ¹⁄₂₈₀₀ to the ¹⁄₇₀ of an inch in diameter, are found in the sea, but chiefly in ponds inhabited by fresh-water plants. They move irregularly over the surface of the water, slowly and continually changing their form by stretching out portions of their gelatinous mass in blunt finger-like extensions, and then drawing the rest of it into them; thus causing the whole mass to change its place. Before it protrudes these pseudopodia or false feet, there is a rush of the internal semi-fluid matter to the spot, due to the highly contractile power of the diaphane, which is often so thin and transparent as to be scarcely perceptible.

When the creature in its progress meets with a particle of food, it spreads itself over it, draws it into its mass, within which a temporary hollow or vacuole is made for its reception; there it is digested, the refuse is squeezed out through the external surface; the nutritious liquid that is left in the vacuole seems to be dispersed in the sarcode, for the vacuole disappears. An Amœba often spreads itself over a Diatom, draws it into a vacuole newly made to receive and digest it; the siliceous shells of the diatom are pushed towards the exterior, and are ultimately thrust out; then the vacuole disappears, either immediately or soon after. These improvised stomachs are the earliest form of a digestive system.

Besides the vacuoles of which there may be several at a time, the slow and nearly rhythmical pulsations of a vesicle containing a subtle fluid may be seen, which changes its position in the interior of the sarcode with every motion of the Amœba. It gradually increases in size, then diminishes to a point, and as some of the digestive vacuoles nearest the surface of the animal are observed to undergo distension when the vesicle contracts, and to empty themselves gradually as it fills, Dr. Carpenter thinks it can hardly be doubted that the function of the vesicle is to maintain a continual movement of nutritious matter, among a system of channels and vacuoles excavated in the substance of the body. It is the first obscure rudiment of a circulating system.

In all the Amœbæ the semi-fluid sarcode, with the numerous bodies suspended in it, rotates at a varied rate within the pellucid coat; a motion presumed to be for respiration, that is to exchange carbonic acid gas for oxygen, so indispensable for animal life.[^[4]]

Although like other animals, the Amœba cannot change inorganic into organic matter, as the vegetable Amœba can do, these two Protozoa are similar in one mode of reproduction; for portions of the animal Amœba or even one of the pseudopodia separate from the gelatinous mass, move to a little distance on the surface of the water, and become independent Amœbæ.

With a high microscopic power, many bodies besides the digesting vacuoles and pulsating vesicles may be seen imbedded in the sarcode of the Amœba princeps; namely, coloured molecules, granules, fat-globules, and nuclei. All these bodies were seen by Mr. Carter, in certain Amœbina he found at Bombay, together with what he believed to be female reproductive cells, and motile particles similar to spermatozoids, or male fertilizing particles.

Fig. 87. Actinophrys sol.—A, ordinary form; B, act of division or conjugation; C, process of feeding; D, discharge of fæcal matter, a and b; o o, contractile vesicles.

The Actinophrys, a genus of the order Radiolaria, differs from the Amœba princeps in having a definite nearly spherical form with slender root-like filamental pseudopodia radiating from its surface in all directions as from a centre. They taper from the base to the apex, and sometimes end in knobs like a pin’s head, but vary much in length and number, and can be extended and retracted till they are out of sight. They are externally of a firmer substance than the sarcode of the body, which is merely a viscid fluid inclosed in a pellucid film. The Actinophrys sol, which is the type of the genus, is a sphere of from ¹⁄₁₃₀₀ to ¹⁄₆₅₀ of an inch in diameter, with slender contractile filaments the length of its diameter extending from its surface as rays from the sun. It can draw them in and flatten its body so as to be easily mistaken for an Amœba. This creature, which is common in fresh-water pools where aquatic plants are growing and even in the sea, has little power of moving about like the Amœba; it depends almost entirely on its pseudopodia for food. They have an adhesive property, for when any animalcule or diatom comes in contact with one of them, they adhere to it; the filament then begins to retract, and as it shortens the adjacent filaments apply their points to the captive, enclose it, coalesce round it, the whole is drawn within the surface of the Actinophrys, the captive is imbedded in the sarcode mass, and passes into a vacuole where it is digested, and then the pseudopodia thrust out the undigested matter by a process exactly the reverse of that by which the food was taken in (D [fig. 87]). The pseudopodia are believed by Professor Rupert Jones to have the power of stunning their prey, for if an animalcule be touched by one of them, it instantly becomes motionless, and does not resume its activity for some time. The pulsations of the contractile vesicle are very regular, and its duty is the same as in the Amœba princeps.

The Actinophryna are propagated like the lowest vegetables by gemmation and conjugation, shown in B [fig. 87]; moreover Mr. Carter saw the production of germ-cells and motile particles in the Actinophrys exactly after the mode already described in the Amœba.

Mr. Carter mentions an instance in which the Actinophrys sol showed what may possibly be a certain degree of instinct. An individual was in the same vessel with vegetable cells charged with particles of starch; one of the cells had been ruptured and a little of the internal matter was protruded through the crevice. The Actinophrys came, extracted one of the starch-grains, and crept to a distance; it returned, and although there were no more starch-grains in sight, the creature managed to take them out from the interior of the cell one by one, always retiring to a distance and returning again, showing that it knew its way back, and where the starch-grains were to be found. On another occasion Mr. Carter saw an Actinophrys station itself close to the ripe spore cell of a plant, and as the young zoospores came out one after another, the Actinophrys caught every one of them even to the last and then retired to a distance as if instinctively conscious that no more remained. Like Amœbæ these animals select their food, but notwithstanding the superior facility and unfailing energy with which they capture prey larger and more active than themselves, they are invariably overcome even by a very small Amœba which they avoid if possible. When they come into contact the Amœba shows unwonted activity, tries to envelope the Actinophrys with its pseudopodia, but failing to capture the whole animal it tears out portions and conveys them to improvised vacuoles to be digested. Dr. Wallich mentions that he had seen nearly the half of a large Actinophrys transferred piecemeal to the interior of its enemy, where it was quickly digested.

Fig. 88, p. 19.

ACANTHOMETRA BULBOSA.

As every part of the body of the Actinophrys is equally capable of performing the part of nutrition, respiration, and circulation; and as in the absence of muscles and nerves they may be presumed to have no consciousness, the marks of apparent intelligence can only be attributed to a kind of instinct, and their motions to the vast inherent contractility of the sarcode and its enclosing film, which is also the case with the Amœbæ.

The Acanthometræ (see [fig. 88], Acanthometra bulbosa) are all marine animals; their skeleton consists of a number of long spicules which radiate from a common centre, tapering to their extremities. These spicules are traversed by a canal with a furrow at the base through which groups of pseudopodia enter, emerging at the apex. Besides, there are a vast number of pseudopodia not thus enclosed, resembling those of the Actinophrys in appearance and action. The body is spherical, and occupies the spaces left between the bases of the spicules. The exterior film covering the body seems to be more decidedly membranaceous than that of the Actinophrys, but it is pierced by the pseudopodia which radiate through it. This exterior film itself is enclosed in a layer of a less tenacious substance, resembling that of which the pseudopodia are formed. There is a species of Acanthometra (echinoides) extremely common in some parts of the coast of Norway, which, to the naked eye, resembles merely a crimson point.

Fig. 90, p. 20.

DICTYOPODIUM TRILOBUM.

Fig. 91. Podocyrtis Schomburgi.

The Polycystina are an exceedingly numerous and widely dispersed group of siliceous rhizopods. They are inhabitants of the deep waters, having been brought up from vast depths in the Atlantic and Pacific oceans. Their bodies are inclosed in siliceous shells, which have either the form of a thin hollow sphere perforated by large openings like windows, or of a perforated sphere produced here and there into tubes, spines, and a variety of singular projections: so they have many varied but beautiful microscopic forms. The animal which inhabits these shells is a mouthless mass of sarcode, divided into four lobes with a nucleus in each and covered with a thick gelatinous coat. It is crimson in the Eucyrtidium and Dictyopodium trilobum of Haeckel (figs. 89 and [90]): in others, as the Podocyrtis Schomburgi, it is olive brown with yellow globules ([fig. 91]). These creatures extend themselves in radiating filaments through the perforations of their shells in search of food, like their type the Actinophrys sol, to whose pseudopodia the filaments are perfectly similar in form, isolation, and in the slow movements of granules along their borders. The Polycystine does not always fill its shell, occasionally retreating into the vault or upper part of it, as in the Eucyrtidium (fig. 89, frontispiece to vol. i.). Sometimes the shell is furnished with radiating elongations, as in the Dictyopodium trilobum ([fig. 90]). In both of these shells the animal consists of four crimson lobes. These beautiful microscopic organisms are found at present in the Mediterranean, in the Arctic and Antarctic seas, and on the bed of the North Atlantic. They had been exceedingly abundant during the later geological periods; multitudes are discovered in the chalk and marls in Sicily, Greece, at Bermuda, at Richmond in Virginia and elsewhere; in all 282 different fossil forms have been described, grouped in 44 genera.

Fig. 92, p. 21.

AULOCANTHA SCOLYMANTHA.

Fig. 93, p. 21.

ACTINOMMA DRYMODES.

Fig. 94, p. 21.

HALIOMMA ECHINASTER.

In certain Polycystina, the perforations of the shell are so large and so close together, that the sarcode body of the animal appears to be covered by a siliceous net. This connects them with the Thalassicollæ, minute creatures found passively floating on the surface of the sea. Th. morum, which is one of the most simple of the few forms known, has a spherical body of sarcode covered with a siliceous net, through which the pseudopodia radiate in all directions, as in the Actinophrys, but it is studded at regular distances with groups of apparently radiating siliceous spicules.

The Aulocantha scolymantha ([fig. 92]), found by M. Haeckel in the Mediterranean, may be taken as an example of the most general form of the Thalassicolla. The siliceous skeleton of some of the Radiolaria resembles the Chinese ivory toy of ball within ball. That of the Actinomma drymodes ([fig. 93]) consists of three perforated concentric spheres, with six strong spicules attached to the outer surface, perpendicular to one another and prolonged in the interior to the central sphere. Hundreds of finer bristle-like spicules radiate from the surface. The animal is chiefly contained in the central sphere, and from it a perfect forest of fine, long pseudopodia radiate in thick tufts through the apertures of the exterior sphere.

The skeleton of the Haliomma ([fig. 94]) consists of only two concentric spheres. In many species of Haliomma and Actinomma the animals are of the most vivid vermilion or purple colour. Little or nothing is known of the reproduction of these microscopic organisms.

The Actinomma drymodes and the Haliomma are two of the most beautiful microscopic rhizopods discovered by M. Haeckel.

There is a family of fresh-water testaceous rhizopods of which one group secretes its shell and the other builds it. The horny shell secreted by the group of the Arcella presents various degrees of plano-convexity, the convexity in some cases amounting to a hemisphere. They rarely, if ever, have mineral matter on their surface, which is studded with regular but very minute hexagonal reticulations. The aperture or mouth of the shell is small, and invariably occupies the centre of the plane surface, its margins being more or less inverted. The form of the shell is exceedingly varied, sometimes it even has horns indefinite in number, sometimes symmetrical, sometimes not; when its test or covering becomes too small for its increasing size, it quits it, and secretes a new one. The filamental pseudopodia proceed from the mouth of the shell only, and by means of these it creeps about on its mouth in search of food.

Fig. 95. Simple Rhizopods.—A, B, Difflugiæ; C, D, Arcellæ.

The Difflugia build their own shells, which are usually truncated spheres, ovate, or sometimes elongated into the form of a pitcher or flask. The most minute recognisable of these shells is about the ¹⁄₁₀₀₀ of an inch in diameter, but they are constructed with the most perfect regularity. The Difflugia pyriformis or symmetrica has the form of an egg with an aperture at the small end. It is entirely made up of rectangular hyaline plates, arranged with the greatest regularity in consecutive transverse and longitudinal rows, the smaller ones being at the extremities, while the larger ones occupy the central and widest portion of the structure. The inhabitant of this abode is an Amœba with a sarcode body covered with a thin film, from whence it sends off pseudopodia through the mouth of its shell. The Difflugia is propagated by conjugation, but before that takes place it becomes densely charged with chlorophyll-cells and starch-grains. The former disappear during the subsequent changes, and are replaced by a mass of colourless cells full of granules which are supposed to be the elements of a new generation. The embryo or earliest form is a minute truncated sphere, but the animal builds up its habitation very much according to local circumstances.

The greater number of the Difflugiæ secrete a substance which forms a smooth layer in the interior, which the animal covers with sarcode from its mouth, and then it drags itself with its pseudopodia to the particles which it selects, and they adhere to it. The particles selected are invariably mineral matter. ‘The selective power is carried to such an extent that colourless particles—sometimes quartzose, sometimes felspathic, sometimes micaceous—are always chosen.’ ‘The particles seem to be impacted into the soft matter, laid on the exterior in the same way that a brick is pressed into the yielding mortar, and that too, in so skilful a manner as to leave the smallest possible amount of vacant area; whilst in the specimens of Difflugia in which tabular or micaceous particles are used, they are sometimes disposed with such nicety that there is no overlapping, but the small fragments are placed so as to occupy the space left between the larger ones. These excellent architects seem to know that in the valves of the Diatoms are combined the properties best suited to their wants, that is, transparency and form, capable of being easily arranged.’

Both the Difflugia and Arcella are Amœbæ in the strictest sense of the word; their bodies consist of sarcode, which sends out finger-like lobes from the mouth of the shell at one end, while the other end has an adhesive property, which fixes it to the bottom. The nucleus and contractile vesicles are identical in character with those of the Amœbæ, and exhibit the same tendency to subdivision at certain periods of the creature’s history that is witnessed on a large scale in the Amœba proper; and the reproductive process is the same.[^[5]]

The Difflugiæ are found in rivulets and pools containing aquatic plants; the condition of the water and the nature of the soil have a great influence on the form of their shell.

The Euglyphæ is the third group of fresh-water rhizopods. They are extremely minute, and there are no mineral particles whatever on their shells, the axes of which do not coincide with the aperture. The interior of the animal is like that of the Arcella and Difflugia, but it differs from them in as much as the pseudopodia and ectosarc, or external coat, are finely granular, and the whole mass of the body possesses a decided degree of adhesive viscidity. The pseudopodia are filiform, tapering, radiating, and readily coalesce; and ‘as if to compensate for the restricted power of locomotion, compared with that of the Amœba proper, the pseudopodia of the Euglyphæ are much more active. The rapidity with which they admit of being projected outwards, and withdrawn into the shell, is unequalled in any other form, presenting the most wonderful example of inherent contractility in an amorphous animal substance, that is to be met with in either of the great organic kingdoms.’[^[6]]

The order Reticularia, with a very few exceptions, are animals dwelling in calcareous microscopic shells, and differing essentially in constitution from all the preceding Rhizopods. The ectosarc or surface-layer of the sarcode in the Amœba and Actinophrys has so much consistence, that their pseudopodia, which are derived from it, have a decidedly firm outline and never coalesce; whereas in the order Reticularia, the sarcode is merely a semi-fluid protoplasm or colourless viscid fluid, without the smallest surface-layer or film, so that their pseudopodia possess no definiteness either in shape, size or number. Sometimes they are cylindrical, and sometimes form broad flat bands, whilst they are often drawn into threads of such extreme tenuity, as to require a high magnifying power to discern them. They coalesce and fuse into each other so freely and so completely when they meet, that no part of their substance can be regarded as having more than a viscous consistence. Their margins are not defined by continuous lines, but are broken by granules irregularly disposed among them, so that they appear as if torn; and these granules, when the animal is in a state of activity, are in constant motion, passing along the pseudopodia from one end to the other, or passing through the connecting threads of this animated network from one pseudopodium to another, with considerable rapidity, analogous to the movement of the particles in the cells of the hairs of the Tradescantia and other plants.[^[7]]

The sarcode body of the Gromiæ is inclosed in a yellowish brown horny envelope or test of an oval shape, with a single round orifice of moderate size, through which the pseudopodia extend into the surrounding water, some forms of the animal being marine, others inhabitants of fresh water. When the animal is at rest all is drawn within the test, and when its activity recommences, single fine threads are put out which move about in a groping manner until they find some surface to which they may attach themselves. When fixed, sarcode flows into them so that they rapidly increase in size, and then they put forth finer ramifications, which diverging come in contact with those from other stems, and by mutual fusion form bridges of connection between the different branching systems; for the protoplasm spreads over the exterior of the test, and from it pseudopodia extend and coalesce, wherever they meet, so that the whole forms a living network, extending to a distance of six or eight times the length of the body. [Fig. 96] represents the Gromia oviformis with its pseudopodia extended.

Fig. 96. Gromia oviformis.

In the Gromiæ the granular particles in the semi-fluid protoplasm are in constant motion. In the finer filaments there is but one current, and a particle may be seen to be carried to the extremity, and return again bringing back with it any granules that may be advancing; and should particles of food adhere to the filament they take part in the general movement. In the broader filaments two currents carrying particles pass backwards and forwards in opposite directions at the same time, and the network in which these motions are going on is undergoing continual changes in its arrangements. New filaments are put forth sometimes from the midst of the ramifications, while others are retracted; and occasionally a new centre of radiation is formed at a point where several threads meet. The food consists of diatoms and morsels of vegetable matter; but the Gromiæ have no vent, so that the indigestible matter collects in a heap within them. However, as the form of the test is such that the animal cannot increase its size, it leaves it when it becomes too small for its comfort and forms another, and it is supposed to get rid of the effete matter at the same time. The Gromiæ have no nucleus or contractile vesicle.

Class II.—Foraminifera.

The geological importance of the Foraminifera, their intrinsic beauty, the prodigious variety of their forms, their incredible multitude, and the peculiarity of their structure, have given these microscopic organisms the highest place in the class of Rhizopods. The body of these animals consists of a perfectly homogeneous sarcode or semi-fluid protoplasm, showing no tendency whatever to any film or surface-layer. It is inclosed in a shell; and the only evidence of vitality that the creature gives, is a protrusion and retraction of slender threads of its sarcode, through the mouth or pores of the shell, or through both according to its structure. [Fig. 97] shows some of their forms.

By far the greater number of the Foraminifera are compound or many-chambered shells. When young, the shell has but one chamber, generally of a globular form; but as the animal grows, others are successively added by a kind of budding in a definite but different arrangement for each order and genus of the class. When the creature increases in size, a portion of its semi-fluid sarcode projects like a bud from the mouth of its shell. If it be of the one-chambered kind, the bud separates from its parent before the shelly matter which it secretes from its surface consolidates, and a new individual is thus produced. But if the primary shell be of the many-chambered kind, the shelly secretion consolidates over the sarcode projection which thus remains fixed, and the shell has then two chambers, the aperture in the last being the mouth, from which, by a protrusion of sarcode, a third chamber may be added, the new chamber being always placed upon the mouth of its predecessor, a process which may be continued indefinitely, the mouth of the last segment being the mouth of the whole shell.

Fig. 97. Various forms of Foraminifera:—A, Oolina claxata; B, Nodosaria rugosa; C, Nodosaria spinicosta; D Cristellaria compressa; E, Polystomella crispa; F, Dendritina elegans; G, Globigerina bulloïdes; H, Textularia Mayeriana; I, Quinqueloculina Bronniana.

By this process an ovate shell with a mouth at one extremity may have a succession of ovate chambers added to it, each chamber being in continuity with its predecessor so that the whole shell will be straight and rod-like, the last opening being the mouth. If the original shell be globular, and if all the successive gemmæ given out be equal and globular, the shell covering and uniting them will be like a number of beads strung upon a straight wire. Sometimes the successive gemmæ increase in size so that each chamber is larger than the one which precedes it; in this case the compound shell will have a conical form, the primary shell being the apex, and the base the last formed, the aperture of which is the mouth of the whole shell; a great many Foraminifera have this structure. The spiral form is very common and much varied. A series of chambers increasing in size may coil round a longitudinal axis, like the shell of the snail; but if each of the successive chambers, instead of being developed exactly in the axis of its predecessor, should be directed a little to one side, a curved instead of a straight axis would be the result; there is a regular gradation of forms of Foraminifera between these two types. The convolutions are frequently flat and in one plane, but the character of the spiral depends upon the successive enlargement or not of the consecutive chambers; for when they open very wide and increase in breadth, every whorl is larger than that which it surrounds; but more commonly there is so little difference between the segments after the spiral has made two or three turns, that the breadth of each whorl scarcely exceeds that which precedes it.

However varied the forms may be, the mouth of the last shell is the mouth of the whole, either for the time being or finally. For all the chambers are connected by narrow apertures in the partitions between them. Each chamber is occupied by a segment of the gelatinous sarcode body of the animal, and all the segments are connected by sarcode filaments passing through the minute apertures in the partitions between the chambers, so that the whole constitutes one compound creature.

Although the character and structure assumed by the semi-fluid bodies of the known Foraminifera have been determined in most cases with admirable precision, it is still thought advisable to arrange them according to the substance of the shell: consequently they form three natural orders; namely, the Porcellanous or imperforate, which have calcareous shells often so polished and shining that they resemble porcelain; secondly, the Arenaceous Foraminifera, consisting of animals which secrete a kind of cement from their surfaces, and cover themselves with calcareous or siliceous sand-grains; and lastly, the Vitreous and Perforated order, which is the most numerous and highly organized of the whole class, has siliceous shells transparent as glass, but acquires more or less of an opaque aspect in consequence of minute straight tubes which perforate the substance of the shell perpendicularly to its surface, and consequently interfere with the transmission of light.

Order of Porcellanous Foraminifera.

The Miliolidæ constitute the porcellanous order, which consists of twelve genera and many species, varying from a mere scale to such as have chambered shells of complicated structure.

The genus Miliola has minute white shells resembling millet seeds, often so brilliantly polished that they are perfectly characteristic of the porcelain family to which they belong. No Foraminifera are better suited to give an idea of the intimate connection between the shell and its inhabitant than the Miliola, the fundamental type of this genus. The shell is a spiral (I, [fig. 97]), which is made up of a series of half turns arranged symmetrically on its two sides. Each half turn is longer and of greater area than that on the opposite side, so that each turn of the spire has a tendency to extend itself in some degree over the preceding one, which gives a concave instead of a convex border to the inner wall of the chamber. The sarcode body of the Miliola consists of long segments which fill the chambers, connected by threads of sarcode passing through the tubular constrictions of the shell. As the animal grows, its pseudopodia extend alternately now from one end, and now from the other extremity of the spiral, and by them it fixes itself to seaweeds, zoophytes, and other bodies, for these Foraminifera never float or swim freely in the water. The genus Miliola is more extensively diffused than almost any other group of Foraminifera; they are most abundant between the shore and a depth of 150 fathoms, and are occasionally brought up from great depths. Beds of miliolite limestone show to what an extent the Miliola abounded in the seas of the Eocene period; but the type is traced back to the Lias.

The genus Peneroplis is distinguished by a highly polished opaque white shell; its typical form is an extremely flat spire of two turns and a half opening rapidly and widely in the last half whorl. It is strongly marked by depressed bands which indicate the septa or shelly partitions between the chambers in the interior. The polished surface of the shell is striated between and transversely to the bands by parallel platted-looking folds ¹⁄₁₄₀₀ of an inch apart. But the peculiarity of this shell and its congeners is, that the partitions between the chambers in its interior are perforated by numerous isolated and generally circular pores which in this compressed type are in a single linear row. Their number depends upon the length of the partition between the chambers, which increases with the age of the animal and size of the shell. There is but one pore in each of the consecutive partitions from the globular centre to the fourth chamber. From the fourth to the seventh chamber the communication is by two pores; after this the number is gradually increased to three, four, six, &c., up to forty-eight, so that the last segment may send out forty-eight pseudopodia from the mouth of the shell. In its early youth one pseudopodium appears to have been sufficient to find food for the animal, but as the shell increased in size and the segments in number, a greater supply of food was requisite and a greater number of pseudopodia were necessary to fish for it. Moreover when an addition to the shell is required the pseudopodia coalesce at their base and form a continuous segment upon which the new portion of the shell is moulded.

In varieties of the Peneroplis where the spire is less compressed there are sometimes two rows of pores in the partitions between the chambers. The Dendritine variety deviates most from that described. It is characterised by a single large aperture in each partition which sends out ramifications from its edges. The form of these openings depends upon that of the spire; when compressed the aperture is linear and less branched at its edges; but in shells which have a very turgid spire it is sometimes broader than it is long, and much branched; but these extremes are connected by a variety of forms. The shells of this variety of the Peneroplis are strongly marked by the depressed bands and striæ, as in the Dendritina elegans (F, [fig. 97]). The segments of the animal inhabiting these shells must be more intimately connected than in most of the other Foraminifera; and the pseudopodia sent through these large apertures out of the mouth of the shell must be comparatively quite a mass of sarcode. The Dendritinæ are inhabitants of shallow water and tropical seas, while the other members of the genus Peneroplis abound in the Red Sea and the seas of other warm latitudes, especially in the zone of the great laminarian fuci. They do not appear in a fossil state prior to the beginning of the Tertiary period.

The last whorls of some of the compressed spiral Foraminifera of the Porcellanous order so nearly encompass all their predecessors, that the transition from a flat spiral to the Orbitolite with its flat disk of concentric rings is not so abrupt as might at first appear. The gradual change may be distinctly traced in the species of the genus Orbiculina. The exteriors of the shells of the genus Orbitolites have less of the opaque whiteness than many others of its family. In its simplest form it is a disk about the ¹⁄₅₀₀ of an inch in diameter, consisting of a central nucleus surrounded by from ten to fifteen concentric circular rings. The surface is usually plane, though sometimes it is concave on both surfaces in consequence of the rings increasing in thickness towards the circumference. The rings or zones are distinctly marked by furrows on the exterior of the shell, and each of these zones is divided by transverse furrows into ovate elevations with their greatest diameter transverse to the radius of the disk, so that the surface presents a number of ovate elevations arranged in consecutive circles round the central nucleus. The margin of the disk exhibits a series of convexities with depressions between them; in each of these depressions there is a circular pore surrounded by a ring of shell: these pores are the only means the animal possesses of communicating with the water in which it lives.

Fig. 98. Simple disc of Orbitolites complanatus.

Fig. 99. Animal of Orbitolites complanatus.

[Fig. 98] is a horizontal section of the simple Orbitolite showing the internal structure of the disk. A pear-shaped chamber with a circumambient chamber forms a nucleus which is surrounded by series of concentric rings of ovate cavities. The chambers of the nucleus and all the cavities are filled with segments of homogeneous semi-fluid sarcode, which constitute the body of the animal ([fig. 99]). The segments in the rings are connected circularly by gelatinous bands of sarcode extending through passages which connect the cavities laterally. The segments are also connected radially by similar sarcode bands, which originate in the mass of sarcode filling the nucleus, and extend to the pores in the margin of the disk. The cavities of each zone alternate in position with those of the zones on each side of it. The animal sends out its pseudopodia through the marginal pores in search of food, which consists of Diatoms and Desmidiaceæ; they are drawn in, digested without any stomach, and the nutritious liquid is conducted by the gelatinous bands from segment to segment and from zone to zone, even to the innermost recesses of the shell.

It is supposed that during the growth of the Orbitolite, when the animal becomes too large for its abode, its pseudopodia coalesce and form a gelatinous massive coat over the margin of the exterior zone, which secretes a shelly ring with all its chambers and passages, each ring being a mere vegetative repetition of those preceding it. That vegetative property enables the animal to repair its shell or add a part that is wanting. For, if a small portion of a ring be broken off and separated from the living animal, it will increase so as to form a new disk, the want of the central part or nucleus not appearing to be of the smallest consequence; indeed, the central rings are very often imperfect. The sarcode of these animals is red, and although the shell is of a brownish-yellow by transmitted light, it is so translucent that the red tint is seen through it.

The simple Orbitolite has many varieties. Sometimes it begins its life as a spiral which changes to a circular disk as it advances in age. It varies in thickness, and some of its very large varieties may be said to consist of three disks or stories of concentric chambers and many marginal pores instead of one. The upper and base stories of concentric chambers are alike, the intermediate one very different, but the sarcode segments in all the three are so connected as to form a very complex compound animal.[^[8]] Different as this structure is from that of the simple Orbitolite, they are merely varieties of the same species; for it has been shown by Dr. Carpenter that, although many pass their lives in the simple one-storied state, they may change into the complex form at any stage of their growth; and as an equally extensive range of variation has been proved by Professor Williamson and Mr. Parker to prevail in other groups of Foraminifera, the tendency to specific variation seems to be characteristic of that type of animal life, and consequently the number of distinct species is less than they were supposed to be.

The Orbitolites are found in the dredgings of all the warmer seas, in vast multitudes at the Philippine Islands, but those from Australia are the most gigantic, being sometimes the size and thickness of a shilling.

Order of Arenaceous Foraminifera.

In the numerous family of Lituolidæ the abode of the animal consists of a cement mixed with very fine particles of sand with larger ones imbedded in the surface. The order includes a wide range of forms divided into three genera, the simplest of which consists of a cylindrical tube twisted into a spiral gradually increasing in diameter, and attached to a foreign substance by one of its surfaces. The creature which lives in it is a uniform cord of sarcode, which sends its pseudopodia out through a large aperture at the extremity of its tube in search of food. Although the tube consists of sand imbedded in an ochreous-coloured cement secreted by the animal, its surface is smooth as a plastered wall. The spiral tubes of this genus take various forms, and in some cases are divided into chambers.

The members of the genus Lituola exude from their surfaces a thick coat of cement with a quantity of siliceous particles roughly imbedded in it, but in some instances the particles are so uniform in size and shape, and are so methodically arranged, that the surface resembles a tesselated pavement. The usual form of the Lituola is a mere string of oval convex chambers increasing gradually in size, and fixed to shells and corals by their flat surfaces. In some instances the shells, or rather the substitutes for shells, take a nautiloid form, and become detached from the foreign bodies to which they were attached. In the highest forms of this genus the chambers are divided by secondary partitions.

The typical form of the genus Valvulina is a three-whorled, three-sided pyramidal shell, with three chambers in every turn of the spire. The aperture is large and round, with a valve of smaller size attached by a tooth of shell to its rim. The creature itself has an exceedingly thin perforated vitreous shell, covered by an incrustation of calcareous particles, which so entirely blocks up the perforations that it can only extend its pseudopodia through the mouth of its shell.

Order of Vitreous Foraminifera.

Nearly all the Foraminifera on the British coasts belong to the Vitreous or Perforated order, which consists of three natural families and many genera. Their shells are vitreous, hyaline, and generally colourless, even although the substance of the animal is deeply coloured; in some species both the animal and its shell are of a rich crimson. The glassy transparency of the shells would be perfect were they not perforated by numerous tubes running from the interior of the chambers straight through the shell, and ending in pores on its surface. According to microscopic measurement the tubes in the Rotalia, which are the largest, are on an average the ¹⁄₁₀₀₀ of an inch in diameter, and as they are somewhat more than that apart, the transparency of the shell appears between them and gives the surface a vitreous aspect. The pseudopodia of the animal have been seen to pass through every part of the wall of the chambers occupied by it; the apertures of the tubuli in this case are wide enough to permit particles of food to be drawn into the interior of the shell. But threads of sarcode of extreme tenuity alone could pass through the tubuli of the Operculina, which are not more than the ¹⁄₁₀₀₀₀ of an inch in diameter, and the distance between them not much greater, which gives the shell an opaque appearance. Particles of food can hardly be small enough to pass through such tubes into the interior to be digested. Dr. Carpenter, however, is almost certain, from the manner in which the animal repairs injuries done to its shell, that the semi-fluid sarcode extends itself at certain times, if not constantly, over the exterior of the shell, as in the Gromia; and therefore it is by no means impossible that the digestive process may really be performed in this external layer, so that only the products of digestion may have to pass into the portion of the sarcode occupying the body of the shell.

In such many-chambered shells as are pierced by tubuli wide enough to permit particles of food to be drawn into the interior, each segment of the animal, being fed within its own chamber, has a life of its own, at the same time that it shares with all the others in a common life maintained by food taken in through the mouth of the shell. There are many instances of this individual life combined with a common life among the lowest tribes of animals.

Although the Perforated order contains types widely apart, they are always connected by intermediate forms; but there is no such connection between the two great natural orders, which are not only separated by the tubuli in the shell, but in many instances by the structure of the interior and the corresponding character of the animal.

In the Lagenidæ, which form the first family of the Perforated order, the vitreous shell possesses great hardness, and is pierced by numerous small tubuli. It is very thin, and of glossy transparency. The first four shells in [fig. 97] represent some of its forms.

The genus Nodosaria has a very extensive range of forms, from the elongated structure to the nautiloid spiral, depending upon the relative proportions and arrangement of the segments. The segments are separated by constrictions transverse to the axis of growth, or by bands as in the Nodosaria rugosa, B, [fig. 97]. It frequently happens that parts of the shell are not perforated; and there are generally longitudinal ribs which sometimes have spines projecting from every part of the interior, as in Nodosaria spinicosta, C, [fig. 97].

In the genus Nodosaria, the axis of growth changes from a straight line to that of a spiral, so that the septa or divisions between the segments cross the axis obliquely, and the aperture instead of being exactly central becomes excentric. Between these extremes there is a numerous series of gradations. The Cristallaria is the highest type; the form is a nautiloid spiral, more or less compressed (D, [fig. 97]), of which each whorl has its chambers extended by winged projections so as to reach the centre, and entirely encloses the preceding whorl. The number of chambers in each whorl is much smaller than in most of the nautiloid spirals, not being more than eight or nine. The divisions are always strongly marked externally by septal bands, varying in character according to the species. The margin of the shell runs into a keel, which is sometimes extended into a knife-edge. Nearly all the Lagena family are found in the North Atlantic and Mediterranean, especially in the Adriatic, which is rich in species. In the Nodosaria the cells which compose the shell have so little connection one with another that they may be easily detached; which gives reason to believe that the separation of the parts may be a means of reproduction and dispersion.

The Globigerinidæ are the most numerous family of the perforated series, and the most remarkable in the history of the existing Foraminifera. They are distinguished by the coarseness of the perforations in their shells, and by the crescentic form of the aperture by which the chambers communicate with each other.

The genus Globigerina consists of a spiral aggregation of globose segments, which are nearly disconnected from each other although united by mutual cohesion. The segments are always somewhat flattened against one another in their planes or junctions, and sometimes the flattening extends over a pretty large surface as in G, [fig. 97]. The entire series of segments shows itself on the upper side, but on the lower side only the segments forming the latest convolution are prominent; they are usually four in number, and are arranged symmetrically round a deep depression or vestibule; the bottom of which is formed by the segments of the earlier convolutions. In this vestibule each segment opens by a large crescent-shaped orifice, the several chambers having no direct communication with each other. The entire shell of the ordinary type may attain the diameter of about ¹⁄₃₀ of an inch, but it is usually much smaller; the typical form, however, is subject to very considerable modifications. In newly formed segments of Globigerina, the hyaline shell substance is perforated by tubuli varying from ¹⁄₁₀₀₀₀ to ¹⁄₅₀₀₀ of an inch in diameter, arranged at pretty regular distances; but in deep seas the surface of the shell is raised by an external deposit into tubercles or ridges, the orifices of the pores appearing between them.

Fig. 100, p. 41.

ROSALINA ORNATA.

Each chamber of the shell is occupied by a reddish-yellow segment of sarcode, from which pseudopodia are seen to protrude; and it is supposed that the sarcode body also fills the vestibule, since without such connecting band it is difficult to understand how the segments which occupy the separate chambers can communicate with each other, or how new segments can be budded off. In the Globigerina the slight cohesion gives reason to believe that the separation of the parts may be a means of reproduction.

The Rosalina ornata, one of the most beautiful specimens of this group, and remarkable for the size of its pores, is represented in [fig. 100] with its pseudopodia extended, and coalescing in some parts.

The shells of the genus Textularia consist of a double series of chambers disposed on each side of an axis, so that they look as if they were mutually interwoven. As the segments for the most part increase gradually in size, the shell is generally triangular, the apex being formed of the first segment, and its base of the two last (H, [fig. 97]).

The aperture is always placed in the inner wall of each chamber, close to its junction with the preceding segment on the opposite side. In the compressed shells it is crescent-shaped, but it is semilunar in the less compressed, and may even be gibbous. The shell is hyaline, with large pores not very closely set, though in some varieties they are minute and near to one another. Sometimes the pores open on the surface in deep hexagonal pits. The older shells are frequently incrusted with large coarse particles of sand, and some specimens from deep water are almost covered with fine sand, but with a good microscope the pores may be seen between them.

The sarcode segments of the animal perfectly correspond in shape and in alternate arrangement with the segments of the shell, and are connected by bands of sarcode passing through the crescent-shaped apertures by which each chamber communicates with that which precedes and follows it.

The Textulariæ are among the most cosmopolitan of Foraminifera; some of their forms are found in the sands and dredgings from all shores, from shallow or moderately deep water. In time they go back to the Palæozoic period.

The Rotalia Beccarii, common on the British coast, affords a good example of the supplemental skeleton, a structure peculiar to some of the higher vitreous Foraminifera. It has a rather compressed turbinoid form with a rounded margin. Its spire is composed of a considerable number of bulging segments gradually increasing in size, disposed with great regularity, and with their opposed surfaces closely fitted to each other. The whole spire is visible on the exterior, with all its convolutions, and on account of the bulging form of the segments, their lines of junction would appear as deep furrows along the whole spire, were they not partly or wholly filled up with a homogeneous semi-crystalline deposit of shell-substance, which is very different in structure and appearance from the porous shell wall of the segments.

The genus Calcarina is distinguished by a highly developed intermediate skeleton with singular outgrowths, which is traversed by a system of canals; through these the animal sends its pseudopodia into the water for food to nourish the whole.

A homogeneous crystalline deposit invests almost the whole of the minute spiral shell of a Calcarina, and sends out many cylindrical, but more generally club-shaped spines in all directions, though they usually affect more or less that of the equator, as in the typical form Calcarina calcar, which is exactly like the rowel of a spur. The spines are for the most part thick and clumsy, and give the shell a very uncouth appearance, especially when their extremities are forked. The turbinoid spire of the shell has a globose centre surrounded by about five whorls progressively increasing in size, and divided by perforated septa into chambers. Each whorl is merely applied to that preceding it, and does not invest it in the least degree. Internally the turns of the spire are separated from each other by the interposition of a solid layer of shell-substance quite distinct from the walls of the chambers. A crystalline deposit begins at the very centre of the spire in a thin layer gradually increasing in thickness as it proceeds, and sending off club-shaped spines from time to time so that the spines are of later and later production, and become thicker and longer. From this it is evident that the intermediate skeleton grows simultaneously with the turns of the spire, but strange as it may seem, their growth is independent, though both are nourished and increased by the sarcode in the interior of the chambers. For the intermediate skeleton is traversed in every part by an elongated network of canals, which begin from irregular lacunæ or openings in the walls of the chambers, and extend to the extremities of the spines. Through these canals threads of the sarcode body of the animal within the chambers have access to the exterior, and provide nourishment for the intermediate skeleton; while pseudopodia, passing into the water through pores in the last partition of the shell, provide for its growth and procure nourishment for the animal. The communication between the adjacent chambers in the whorls, is by means of a series of pores in the septa, or partitions; and it is through the pores of the last septum that the pseudopodia of the animal have access to the water to provide for the growth of the spire, for the punctures on the surface are merely the terminations of some of the branching canals. On approaching the surface the canals become crowded together in some parts, leaving columns of the shelly skeleton unoccupied which either appear as tubercles on the surface, or, if they do not rise so high, form circular spots surrounded by punctations which are the apertures of the canals.

The Rotaline series of the Globigerina family is one of the most numerous and varied of the whole class of Foraminifera; but varied as their forms are, they all bear the characteristic marks which distinguish their order, with this essential difference, that in the genus Globigerina each chamber of the spire has a communication with the central vestibule by a crescent-shaped aperture, while in the Rotalinæ each chamber only communicates by a crescentic aperture with that which precedes and follows it.

In the Rotaline group the internal organization rises successively from the simple porous partition between the chambers, to the double partition with the radiating passages, and from the latter to the double partitions, intermediate skeleton, and complicated system of canals. To these changes the structure of the compound animal necessarily corresponds, for it may be presumed that not only the chambers but all the passages and canals in the interior of the shell are either permanently or occasionally filled with its sarcode body.

However, it is in the Nummuline family that the Foraminifera attain the highest organization of which they are capable. This family surpasses all the Vitreous tribe in the density and toughness of the shell, the fineness of its tubuli, and in the high organization of its canal system. Their forms vary from that resembling a nautilus or ammonite to a flat spiral or cyclical disk, like an Orbitolite, though vastly superior to it in organization both with regard to the animal and to the structure of the shell.

All the species of the genus Nummulite are spiral; in the typical form the last turn of the spire not only completely embraces, but entirely conceals, all that precede it. In general, the form is that of a double convex lens of more or less thickness; some are flat, lenticular, and thinned away to an acute edge, while others may be spheroidal with a round, or obtuse edge. They owe their name to their resemblance to coins, being, in general, nearly circular. Their diameters range from ¹⁄₁₆th of an inch to 4¹⁄₂ inches, so that they are the giants of their race; but the most common species vary from ¹⁄₂ an inch to 1 inch in diameter.

Fig. 101. Section of Faujasina.

[Fig. 101] represents a section of the Nummulite Faujasina near and parallel to the base of the shell. It shows a series of chambers arranged in a flat spiral, and increasing in size from the centre to the last turn of the spire, which embraces and conceals all that precede it. Every segment of the animal is enclosed in a shell of its own, so that they are separated from one another by a double wall and space between; however, they are connected in the spiral direction by narrow passages in the walls.

The segments of the animal in the exterior whorl have direct communication with the water by means of a shelly marginal cord, a, [fig. 101], perforated by multitudes of minute tubes, less than the ¹⁄₁₀₀₀₀ of an inch in diameter, through which threads of sarcode finer than those of a spider’s web can be protruded. These tubuli are so very fine and numerous, that they characterize the Nummuline family.

Fig. 102. Interior of the Operculina.

[Fig. 102] represents the interior of the Operculina, which is an existing representation of the Nummuline type. Every segment of the animal is enclosed in a shell of its own, but all the segments are connected in the spiral direction by narrow passages in the walls as in the Faujasina.

Although each of the interior whorls has its perforated marginal band, the segments can have no direct access to the water; however, they are indirectly brought into contact with it by means of a system of branching shelly canals, radiating from the central chamber, ending in conspicuous pores in the external surface of the shell. During this course the canals send small tubes into the chambers on each side of them; through these the internal segments of the animal can fill the canals with cords of sarcode, and protrude them into the water, whence they are supplied with food.

The genus Polystomella is distinguished by the high development of the intermediate skeleton and the canal system that maintains it. The Polystomella crispa ([fig. 97], E), a beautiful species common on the British coasts and in other temperate seas, has a lenticular form, the ¹⁄₁₆ to the ¹⁄₁₂ of an inch in diameter. It consists of a small number of convolutions winding round the shorter axis of the lens, increasing rather rapidly in breadth, and each one almost entirely enclosing its predecessor, so that the shell is exactly alike on both sides, and only the last convolution is to be seen. At the extremities of the axis there is a mass of solid shell-substance, perforated by orifices which are the apertures of a set of straight, parallel canals. In the figure only the last convolution is visible, upon which the convex septal bands are very conspicuous, dividing the surface into well marked segments, upon the exterior edge of each of which there are strong transverse crenulations. The only communication which the chambers have with the exterior, is by means of a variable number of minute orifices near the inner margin of the sagittate partition-plane, close to its junction with the preceding convolution; a very high microscopic power is required to see them, as well as the minute tubercles with which the surface of the shell is crowded, more especially on the septal bands and in the rows of depressions between the segmental divisions.

The sarcode animal itself corresponds exactly with the form and spiral arrangement of the chambers so strongly marked on the exterior of the shell. The segments form a spiral of crescents, smooth on the convex and crenulated on the concave side; and from the latter threads of sarcode proceed, which pass through pores in the inner margins of the partitions, and unite them into one animal.

The Polystomella lives in tropical seas; P. crispa in temperate latitudes, and P. striato-punctata inhabits the polar waters; the genus is found everywhere.

Although variety of form without specific difference is characteristic of the Foraminifera, it sometimes happens that identity of external form is accompanied by an essential difference in internal structure. Of this the Cycloclypeus is an instance; it is a rare species of nummuline, dredged up from rather deep water off the coast of Borneo. The shell is gigantic, some specimens being two and a half inches in diameter; but its mode of growth is the same with that of the most complicated Orbitolite. It consists of three superposed stages of circular discs, each circle of chambers enclosing all those previously formed. However, each segment of the animal being enclosed in its own shelly envelope, a supplemental skeleton, and a radial, vertical and annular system of canals, prove that the two animals belong to essentially different families of Foraminifera. There are many instances, especially in the Rotaline group, of isomorphism accompanied with generic difference; thus no reliance can be placed on variety of external form, unaccompanied by change of internal structure.

An attempt has been made in the preceding pages to describe a few species most characteristic of some of the genera of this multitudinous class; and of those selected a mere sketch of the most prominent features of the animal and its abode is given, that some idea may be formed of the wonderfully complicated structure of beings, which are mostly microscopic specks. Yet the most minute circumstances in the forms of the animals and their shells, with their varieties and affinities, have been determined with an accuracy that does honour to microscopic science.

They are now arranged in a natural system by William B. Carpenter, M.D. F.R.S. assisted by William K. Parker, Esq., and T. Rupert Jones, Esq., and published in the Transactions of the Ray Society in 1862. To this admirable work, the author is highly indebted.

It was known that different types of Foraminifera abound at different depths on the coasts of the ocean; but it was long believed that no living creature could exist in its dark and profound abyss. By deep-sea sounding, it has been ascertained that the basin of the Atlantic Ocean is a profound and vast hollow or trough, extending from pole to pole; in the far south, it is of unknown depth, and the deepest part in the north is supposed to be between the Bermudas and the Great Banks of Newfoundland. But by a regular series of soundings made by the officers of the navies of Great Britain and the United States, for the purpose of laying a telegraphic cable, that great plain or steppe was discovered, now so well known as the telegraphic plateau, which extends between Cape Race in Newfoundland, and Cape Clear in Ireland. From depths of more than 2,000 fathoms on this plateau, the ooze brought up by the sounding machine consisted of 97 per cent. of Globigerinæ. The high state of preservation of these delicate shells was no doubt owing to the perfect tranquillity which prevails at great depths; for the telegraphic plateau and the bed of the deep ocean everywhere is covered by a stratum of water unruffled by the commotion raised by the hurricane which may be raging on the surface. The greater number of the Globigerinæ were dead empty shells; but although in many the animal matter was quite fresh, Professor Bailly of New York could not believe that such delicate creatures could live on that dark sea bed, under the pressure of a column of water more than 2,000 fathoms high, a weight equal to rather more than that of 340 atmospheres or 5,100 lbs. on every square inch of sea-bed; wherefore he concluded that the tropical ocean and the Gulf Stream, which absolutely swarm with animal life, must have been the birth-place and home of these minute creatures, and that this mighty ‘ocean river,’ which divides at the Great Banks of Newfoundland, and spreads its warm waters like a fan over the north Atlantic, deposits their remains over its bed, which has thus been their grave-yard for unknown periods, and which, in the lapse of geological time, may be raised above the waves as dry land.

Professor Ehrenberg on the contrary concluded that residentiary life exists at the bottom of the ocean, both from the freshness of the animal matter found in the shells, and from the number of unknown forms which are discovered from time to time at various and often great depths along the coasts. This opinion has been confirmed beyond a doubt on several occasions, especially by Dr. Wallich, who accompanied an expedition sent under the command of Sir Leopold M‘Clintock, to sound the North Atlantic for laying a telegraphic line.

In doing that two operations are requisite. The first is to ascertain the depth: when that is known, the nature of the sea-bed must be determined, and on that account a sample of it is then sounded for; but owing to the difficulty of ascertaining the exact time at which the ground is struck, a quantity of rope in excess of the depth is given out, which lies on the bottom of the sea while the machine is being drawn up, which occupies a considerable time when the depth is great. About midway between Greenland and the north of Ireland, when the machine was hauled up from a depth of a mile and a half, several starfish were clinging with their long spiny arms to fifty fathoms of the rope that had been lying on the surface of the sea-bed while the machine was being drawn up, and to that part of the rope alone. They continued to move their limbs energetically for more than a quarter of an hour after they were out of the water. They certainly had not been entangled in the line while swimming, because star-fishes are invariably creeping animals. The deposit on which they had rested at the bottom of the ocean contained ninety-five per cent. of Globigerinæ. Abundance of these minute Foraminifera were found in the stomachs of the starfish; which seemed to prove not only that the starfish were caught on their natural feeding ground, but that their food was living organisms whose normal abode is the surface of the bed of the deep ocean.

Dr. Wallich also discovered in the ooze brought up from a depth of nearly two miles and a quarter a number of small bodies from ¹⁄₁₆ to ¹⁄₄ of an inch in length and about a line in breadth. They consisted of equal globes arranged in a straight line like the Nodosaria, or built up, each lying on part of the one below it, and increasing in size from the uppermost about ¹⁄₁₂₅₀ to the undermost about ¹⁄₄₅₀ of an inch in diameter. Both of these forms, called coccospheres, consisted of sarcode enclosed in a calcareous deposit; and were studded at nearly regular distances by minute round or oval bodies concave below, and with an aperture on their convex surface sometimes single, sometimes double. These coccospheres were also found free in the ooze, and had been seen previously by Capt. Dayman. They have likewise been seen as free organisms living on the surface of the ocean.

The ooze in the bed of the Atlantic ocean, as well as of the Mediterranean and Adriatic contains fifty per cent. of Globigerinæ; they exist in the Red Sea, in the vicinity of the West Indian Islands, on both sides of South America and near the Isle of France, but not in the Coral Sea which is occupied by different genera. Though in utter darkness, at the bottom of a deep ocean, these little creatures can procure food by means of their pseudopodia, whose extreme sensibility makes up for the want of sight; and the very excess of pressure under which they live insures them a supply of oxygen at depths to which free air cannot penetrate, for it is believed that the quantity of dissolved air that water contains is in proportion to the pressure.

Fossil Foraminifera enter so abundantly into the sedimentary strata, that Buffon declared ‘the very dust had been alive.’ 58,000 of these fossil shells have been computed in a cubic inch of the stone of which Paris and Lyons are built. The remains of these Rhizopods are for the most part microscopic. M. D’Orbigny estimated that an ounce of sand from the Antilles contained 1,800,000 shells of Foraminifera. A handful of sand anywhere, dry sea-weeds, the dust shaken from a dry sponge, are full of them.

When the finer portions of chalk amounting to one half or less are washed away, the remaining sediment consists almost entirely of the shells of Foraminifera, some perfect, others in various stages of disintegration. In some of the hard limestones and marbles, the relics of Foraminifera can be detected in polished sections and in thin slices laid on glass. It is now universally admitted that some crystallized limestones which are destitute of fossil remains, had been originally formed by the agency of animal life, and subsequently altered by metamorphic action; the opinion is gradually gaining ground among geologists that such is the history of the oldest limestones.

At certain geological periods circumstances favoured the development of an enormous multitude of individual animals. In the earlier part of the Tertiary period the Nummulites acquired an extraordinary size. They were like very large coins two or more inches in diameter, and were accumulated in such quantities as to constitute the chief part of the nummulitic limestone; a formation in some places 1,500 feet thick, which extends through southern Europe, Libya, Egypt, Asia Minor, and is continued through the Himalayan mountains into various parts of the Indian peninsula, where it is extensively distributed. The Great Pyramid of Egypt is built of this limestone, which gave rise to singular speculations with regard to the Nummulites in very ancient and even in more recent times. Although this is incomparably the greatest, it is by no means the only instance of an accumulation of the fossil shells of individual animals. The ‘Lingula flags,’ a stratum in the upper Cambrian series of North Wales, was so named from the abundance of the Brachiopod Lingula that it contains.

Professor Ehrenberg discovered that the shells of the Foraminifera sometimes undergo an infiltration of silicate of iron, which fills not only the chambers, but also their canal-system even to its minutest ramifications, so that if the shell be destroyed by dilute acid, a perfect cast of the sarcode matter remains. The greensands in the different geological strata from the Silurian formation upwards, are chiefly composed of these casts; and Professor Baily of the United States more recently discovered that a process of infiltration is even now taking place in some parts of the ocean bed, and that beautiful casts of Foraminifera may be obtained by dissolving their shells with dilute acid.

A most extensive comparison of the Foraminiferous group of Rhizopods, recent and fossil, has been made by Messrs. Parker and Rupert Jones from almost every latitude on the globe, from the arctic and tropical seas, from the temperate zones in both hemispheres, and from shallow as well as deep-sea beds. They have also reviewed the fossil Foraminifera in their manifold aspects as presented by the ancient geological faunas throughout the whole series from the Tertiary down to the Carboniferous strata inclusive; and have come to the astonishing conclusion that scarcely any of the species of the Foraminifera met with in the secondary rocks have become extinct. All that they had seen have their counterparts in the recent Mediterranean deposits. Throughout that long series of geological epochs even to the present day, the Foraminifera show no tendency to rise to a higher type; but variety of form in the same species prevailed then as it does now.

Subsequently to this investigation, a gigantic Orbitulite twelve inches in diameter, and the third of an inch thick, has been found in the Silurian strata in Canada. The largest recent species Dr. Carpenter had seen was about the size and thickness of a shilling.

The lowest stratum of the Cambrian formations has been regarded as the most ancient of the Palæozoic rocks; now, however, strata of crystallized limestone near the base of the Laurentian system, which is 50,000 feet thick in Canada, are discovered by Sir W. E. Logan to have been the work of the Eozoön Canadense, a gigantic Foraminifer, at a period so inconceivably remote that it may be regarded as the first appearance of animal life upon the earth. In a paper published by Dr. Carpenter, in May 1865, he expressed his opinion that the Eozoön would be found in the older rocks of central Europe; and in the December following he received specimens from the fundamental quartz rocks of Germany, in which he found undoubted traces of the Eozoön. Here the superincumbent strata are 90,000 feet thick; the transcendent antiquity of the Eozoön is therefore beyond all estimation.

The fossil Eozoön consists of a succession of parallel rows or tiers of chambers, in which the sarcode of the living animal had been replaced by a siliceous infiltration, so that when the calcareous shell was destroyed by dilute acid, the cast was found to be precisely like that of a Nummulite; thin slices of it taken in different directions being examined with a microscope, it was found that the siliceous matter had not only filled that portion of the chambers which had been occupied by the sarcode-body of the animal and the canal-system, but had actually taken the place of the pseudopodial threads, the softest and most transitory of living substances, which were put forth through tubuli in the shell-walls of less than the ¹⁄₁₀₀₀₀ part of an inch in diameter. ‘These are the very threads themselves turned into stone by the substitution which took place, particle by particle, between the sarcode body of the animal and certain constituents of the water of the ocean, before the destruction of the sarcode by ordinary decomposition.’[^[9]] The shell had an intermediate skeleton, but the minute tubes in the walls of the chambers are so characteristic of the Nummulites, that they were sufficient alone to determine the relationship of the Eozoön to its modern representative.

The external shape and limits to the size of the individual Eozoön have not been determined with certainty, on account of its indefinite mode of growth, and the manner in which the fossilized masses are connected with the highly crystalline matrix in which they are imbedded; there is no doubt, however, that they spread over an area of a foot or even more, and attained a thickness of several inches. As they seem to have increased laterally by buds which never fell off, they formed extensive reefs; at the same time they had a vertical growth, for in some of the reefs the older portions appear to have been fossilized before the newer were built up on them as a base, exactly like the coral reefs in the tropical ocean of the present day,[^[10]] with this difference however, that shells and other crustaceans are associated with the corals, while no organic body has been found in the Eozoön reefs; nevertheless the Eozoön must have had food. It may therefore be inferred that parts at least of that primeval ocean swarmed with animal life, whose remains have been obliterated by metamorphic action. Carbon (which in the form of graphite both constitutes distinct beds, and is disseminated through the siliceous and calcareous strata of the Laurentian series, as well in Norway as in Canada), may indicate the existence of vegetation in the Eozoön period.

The Eozoön is by no means confined to Canada and central Europe. The serpentine marble of Tyree which forms part of the Laurentian system on the west of Scotland, and a similar rock in Skye, when subjected to minute examination, are found to present a structure clearly identical with that of the Canadian Eozoön. And the like structure has been discovered by Mr. Sanford in the serpentine marble of Connemara, known as Irish green. The age of that rock however, is doubtful: for when it was discovered to contain Eozoön, Sir Roderick Murchison who had previously studied its relations was at first inclined to believe it belonged to the Laurentian series; now however, he considers the Connemara marble to be of the Silurian age. ‘If this be the case it proves that the Eozoön was not confined to the Laurentian period, but that it had a vast range in time, as well as in geographical distribution; in this respect corresponding to many later forms of Foraminifera which have been shown by Messrs. Parker and Rupert Jones to range from the Trias to the present epoch.’[^[11]]

The Carpenteria found in the Indian seas forms a link between the Foraminifera and Sponges. The shell is a minute cone adhering to the surface of corals and shells, by its wide base which spreads in broad lobes. Double-walled chambers and canals form a spiral within it, and are filled with a spongy sarcode of a more consistent texture than the sarcode of the Foraminifera, which in the larger chambers is supported by siliceous spicules similar to those which form the skeletons in sponges.

Class III.—Sponges.

According to the observations of Mr. Carter, sponges begin their lives as solitary Amœbæ which grow by multiplication into masses, and assume endless forms according to the species; turbinate, bell-shaped, like a vase, a crater, a fan, flat, foliaceous and lobed or branching and incrusting the surface of stones. All the Amœbæ are so connected as to form one compound animal. The whole substance of a sponge is permeated by innumerable tubes which begin in small pores on the surface, and continually unite with one another as they proceed in their devious course to form a system of canals increasing in diameter and ending in wide openings called oscula, on the opposite side of the mass. Currents of water enter through the pores on the surface, and bring minute portions of food which are seized upon by a vast multitude of Amœbæ with long cilia which form the walls of the tubes and canals; and after they have extracted the nutritious part, the offal is carried into the sea through the oscula, by the current of water whose flux is maintained by the vibrations of the cilia. In the compressed and many of the tubular sponges the water passes through them in a straight line; in branched and encrusting sponges, the afferent and efferent openings are on the same surface. The water is inhaled continuously and gently like an animal breathing, but it is rapidly and forcibly ejected; and in its passage it no doubt furnishes oxygen to aërate the juices of the compound animal, whose flesh or sarcode is irritable while alive, and which has the power to open and shut the pores and oscula of the canals, for the whole sponge forms one compound creature whose mass is nourished by the myriads of Amœbæ of which it is constituted.

Within the animated sarcode mass of the sponges there is in most cases a complicated skeleton of fibrous network, either horny, calcareous, or siliceous, which supports the soft mass, and determines its form.

Besides the skeleton, the mass of sponges is for the most part strengthened and defended by siliceous, and more rarely by calcareous, spines or spicules, either imbedded among the fibres of the skeleton, or fixed to them by their bases. The fibres of the skeleton network always unite, whether they be horny, calcareous, or siliceous; the spicules never, though they often lie in confused heaps over one another. They are of innumerable forms and arrangements. Some are like long needles lying close together in bundles, pointed or with a head like a pin at one or both ends; a great number are stellate with long or short rays; there may even be several different forms in the same sponge. Many calcareous sponges have cavities full of organic matter; and when the calcareous matter is dissolved by dilute acid, the organic base is left.

The common commercial sponges have a skeleton which consists of a network of tubular, horny, tough, and elastic fibres which cross in every direction. They have no spicules or very few; and when such do project from the horny skeleton, they are generally conical, attached by their bases, and their surface is often beset with little spines arranged at regular intervals, which gives them a jointed appearance. The common sponge which is so abundant in the Mediterranean has many forms; those from the coast of North America are no less varied, but that most used in the United States is turbinate, concave, soft, and tomentose.

Fig. 103. Section of Sponge.

In the calcareous sponges a mass of three-rayed spicules surround the interior canals, where they are held together by a cartilaginous substance which is wanting in the horny sponges, but which remains in this order after the destruction of the more delicate matter when the sponge is dried.[^[12]] The pores are also occasionally defended by the projecting points of half buried spines.

In nearly every species of this order the pores on the surface are protected by spicules; and they are also projected from the surface of the large cloacal cavity, and curved towards its opening, to defend it from Annelids and other enemies.[^[13]] Some species have a long ciliary fringe at the orifice of the cavity, through which the water may pass out, but no animal can come in.

The spicula and skeleton of most of the marine sponges are siliceous and singularly beautiful; the skeleton of the Dactylocalyx pumiceus of Barbadoes is transparent as spun glass; and a species from Madagascar has numerous simple transparent and articulated spicules implanted in the siliceous fibres of the skeleton. The Cristata, Papillaris, Ovulata, and many more have siliceous skeletons, some garnished with spicules of various forms, and the surface occasionally covered with a layer of siliceous granules.

The variety in the size, structure, and habits of the marine sponges is very great: temperate and tropical seas have their own peculiar genera and species; some inhabit deep water, others live near the surface, while many fix themselves to rocks, sea-weeds, and shells, between high and low water mark. There are very few dead oyster, whelk, scallop, and other shells that escape from the ravages of the Cliona, an extremely minute burrowing sponge of the simplest structure, which has a coat of siliceous spicules supposed to be the tools with which it tunnels a labyrinth through the mid-layer of the shell, in a pattern that varies with the species of the sponge. A communication is formed here and there with the exterior by little round holes, through which the sponge protrudes its yellow papillæ. From the force exhibited by this little sponge, it may perhaps be inferred to possess a rudimentary muscle and nerve.[^[14]]

Sponges are propagated twice in the year by minute ciliated globules of sarcode, detached from the interior of the aquiferous canals, which swim like zoospores to a distance, come to rest, and lay the foundation of new sponges. The little yellow eggs of Halichondria panicea are lodged in the interstices between the interior canals; when mature, they are oval and covered with cilia, and are carried out by the currents; and after swimming about for some days fix on a solid object, become covered with bristles, spread out into a transparent film, charged with contractile vesicles of different sizes in all degrees of dilatation and contraction, as well as with sponge ovules. Spicules are developed at the same time, and these films ultimately become young sponges, and if two happen to meet they unite and are soldered together.[^[15]] Besides eggs, larger bodies covered with radiating spicules are produced, containing granular particles of sarcode, each of which when set free by the rupture of the envelope, becomes an Amœba-like creature, and ultimately a sponge.

Fresh-water sponges are sometimes branched, and sometimes spread over stones, wood, and other substances; and one species covers an earthy mass some inches thick formed by its own decayed matter. The skeleton of such species as have one, consists of bundles of siliceous spicules, held together and mixed with groups of needles, the rods of which project through the surface of the sponge and render it spinous. The motions in the gelatinous sarcode mass are the most remarkable feature in the fresh-water sponges, which all belong to the genus Spongilla. Mr. Carter observed that portions of the surface of some individuals of the Spongilla fluviatilis in his aquarium had long cilia by means of which they rapidly changed their places during the spring, but when winter came they emitted processes from such parts of their surfaces as were free from cilia and retracted them again just like Amœbæ. These portions often had cells, and when the Amœba-like motions ceased, a nucleus and nucleolus appeared within them, and at last the whole gelatinous sarcode mass consisted of these cells or globules. Some had no nucleus, but were filled with green or colourless granules.

At certain seasons of the year, whatever the form of the fresh-water sponges may be, a multitude of minute hard yellow bodies are produced in their deeper parts. They consist of a tough coat containing radiating spicules like a pair of spoked wheels united by an axle with a pore in its surface. Within this last there is a mass of motionless granular cells, and when put into water the cells come out at the pore and give rise to new sponges.

Insulated groups of germs covered with cells called swarm-cells seem to form parts of the sponges; they lie completely within the mass of the living sponge. They have the form of a hen’s egg, are visible to the naked eye, and when they come into the water they swim in all directions for a day or two; become fixed; a white spot within is enlarged; and the constituents of young sponges appear.[^[16]]

The generic forms of fossil sponges augment in number and variety from the Silurian to the Cretaceous beds, where the increase is rapid; but all the sponges which had a stony reticulated form without spicules passed away with the Secondary epoch, so that the family has no representatives in the Tertiary deposits or existing seas. The calcareous sponges which abound in the Oolite and Cretaceous strata, and attain their maximum in the Chalk, are now almost extinct, or are represented by other families with calcareous spicules. Siliceous fossil sponges are particularly plentiful. In England extensive beds of them occur in the Upper Greensand, and in some of the Oolitic and Carboniferous Limestones; and some beds of the Kentish Rag are so full of their siliceous spicules, that they irritate the hands of the men who quarry them. Since every geological formation except the Muschelkalk is found in England, the number and variety of fossil sponges are very considerable. The horny sponges are more abundant now than they were in the former seas. According to M. D’Orbigny the whole number of fossil sponges known and described amount to thirty-six genera and 427 species, which is probably much below the real number.[^[17]]

Class IV.—Infusoria.

The Infusoria, which form the second group of the Protozoa, are microscopic animals of a higher grade than any of the preceding creatures, although they go through their whole lives as isolated single cells of innumerable forms. They invariably appear in stagnant pools and infusions of animal and vegetable matter when in a state of rapid decomposition. Every drop of the green matter that mantles the surface of pools in summer teems with the most minute and varied forms of animal life. The species called Monas corpusculus by the distinguished Professor Ehrenberg, has been estimated to be ¹⁄₂₀₀₀ part of a line in diameter. ‘Of such infusoria a single drop of water may contain 500,000,000 of individuals, a number equalling that of the whole human species now existing upon the face of the earth. But the varieties in size of these animalcules invisible to the naked eye are not less than that which prevails in almost any other natural class of animals. From the Monad to the Loxades or Amphileptus, which are the fourth and sixth part of a line in diameter, the difference in size is greater than between a mouse and an elephant; within such narrow bounds might our ideas of the range in animal life be limited if the sphere of our observation was not augmented by artificial aid.’[^[18]]

This singular race of beings has given rise to the erroneous hypothesis of equivocal or spontaneous generation, that is to say, the production of living animalcules by a chemical or even fortuitous combination of the elements of inert matter. That question has been decided by direct experiment, for Professor Schultz kept boiled infusions of animal and vegetable matter for weeks in air which had passed through a red-hot tube, and no animalcules were formed, but they appeared in a few hours when the same infusions were freely exposed to the atmosphere, which shows clearly that the germs of the lowest grade of animal life float in the air, waiting as spores do, till they find a nidus fit for their development.

M. Pasteur, Director of the Normal School in Paris, in a series of lectures published in the ‘Comptes Rendus,’ has not only proved that the atmosphere abounds in the spores of cryptogamic fungi and moulds, but with infusoria of the form of globular monads, the Bacteria, and vibrios, which are like little rods round at their extremities and extremely active. The Bacteria mona and especially the Bacteria terma, are exceedingly numerous. These minute beings are the principal agents in the decomposition of organic matter. They are more numerous in dry than in wet weather, in towns than in the country, on plains than on mountains.

In a memoir read at the Academy of Sciences, Paris, Mr. J. Samuelson mentions that he had received rags from Alexandria, Japan, Melbourne, Tunis, Trieste and Peru. He sifted dust from the rags from each of these localities respectively through fine muslin into vases of distilled water. Life was most abundant in the vases containing dust from Egypt, Japan, Melbourne, and Trieste. The development of the different forms was very rapid, and consisted of protophytes, Rhizopods and true Infusoriæ. In most of the vases monads and vibrios appeared first, and from these Mr. Samuelson traced a change first into one then into another species of infusoria. In the dust from Japan he followed the development of a monad into what appeared to be a minute Paramœcium, then into Lexodes cucullus, and finally into Colpoda cucullus. From these and other experiments it is proved that many infusoria now classed as distinct types are really one and the same animal in different states of development. That appears to be the case also with the Amœbæ. In the dust from Egypt Mr. Samuelson found a new Amœba whose motions were very rapid; as to shape and mode of motion he compared it to soap bubbles blown with a pipe. He traced the gradual changes of the globular form of this Amœba until its pseudopodia were in full action, its increase by conjugation, and other circumstances of its life. In the same dust and in that only, the development of the Protococcus viridis was seen, and that in such abundance that at last the water was tinged green by that plant. In the dust from Egypt a vibrio was changed into a vermiform segmented infusoria of an entirely new type. Its length varied from the ¹⁄₁₅₀ to ¹⁄₁₀₀ of an inch, each ring was ciliated, and the whole series of cilia extending along the body acted in concert; a circlet of them surrounded the anterior segment; a canal seemed to extend throughout the body. It was propagated by bisection; the two parts remained attached to one another; an independent ciliary motion was observed in each which did not interfere with the motion of the whole. It was supposed to be a larval form or series of forms. Mr. Samuelson’s observations show, that the atmosphere in all the great divisions of the globe is charged with representatives of the three kingdoms of nature, animal, vegetable, and mineral: that the animal germs not only include the obscure types of monads, vibrios, and Bacteria, but also the Glaucoma, Cyclides, Vorticella, and other superior Infusoriæ, and occasionally though very rarely germs of the Nematode worms.

It has been already mentioned that many of the microscopic fungi are ferments, aiding greatly in the decomposition of organic matter. They however are by no means the only agents in decomposition. The moment life is extinct in an animal or vegetable, Infusoria of the lowest grade seize upon the inanimate substance, speedily release its atoms from their organic bond, and restore them to the inorganic world whence they came. The ferment which transforms lactic acid into butyric acid is a species of vibrio which abounds in the liquid, isolated or united in chains; they glide, pirouette, undulate, and float in all directions, and multiply by spontaneous division. Vibrios possess the unprecedented property of living and propagating without an atom of free oxygen; they not only live without air, but air kills them. This singular property forms an essential difference between the Vibrios and the Mycoderms: the former cannot live in oxygen; the latter cannot live without it, and as soon as it is exhausted within the infusion, they go to the surface to borrow it from the atmosphere.

There are also two groups of Infusoria which possess these opposite characters, one being unable to live in oxygen, while the other cannot live without it; sometimes they even inhabit the same liquid. When the tartrate of lime is put into water along with some ammoniacal and alkaline phosphates, a Monad, the Bacteria terma, and other Infusoria appear after a time. These little animals bud rapidly in an infusion of animal matter, then a slight motion is produced by the appearance of the Monas corpusculum and the Bacterium terma, which glide in wavy lines in all directions in quest of the oxygen dissolved in the liquid, and as soon as it is exhausted they go to the surface in such numbers as to form a pellicle, where by aid of the oxygen they form the simple binary compounds water, ammonia, and carbonic acid. In the meantime the Vibrios, which are without oxygen, are developed below, and keep up the fermentation, and between the two, the work of decomposition is completed.

It is not the worm that destroys our dead bodies; it is the Infusoria, the least of living beings. The intestinal canal of the higher animals, and of man, is always filled during life not only with the germs of vibrios, but with adult and well-grown vibrios themselves. M. Leewenhoeck had already discovered them in man, a fact which has since been confirmed. They are inoffensive as long as life is an obstacle to their development, but after death their activity soon begins. Deprived of air and bathed in nourishing liquid, they decompose and destroy all the surrounding substances as they advance towards the surface. During this time, the little Infusoria, whose germs from the air had been lodged in the wrinkles and pores of the skin, are developed, and work their way from without inwards, till they meet the vibrios, and after having devoured them, they perish, or are eaten by maggots.

Of all the Infusoria and ferments the Vibrios are the most tenacious of life; their germs resist the destructive effect of a temperature of 100° Cent. The spores of the Mucedines are still more vivacious; they grow after being exposed to a heat of 120° Cent., and are only killed by a temperature of 130° Cent. As neither spores of the fungi nor the germs of the Infusoria are ever exposed to so high a temperature while in the atmosphere, they are ready to germinate as soon as they meet with a substance that suits them.

M. Ehrenberg has estimated that the Monas corpusculum is not more than the ¹⁄_24,000th part of an inch in diameter; whence Dr. M. C. White, assuming that the ova of the Infusoria and the spores of minute fungi are only the ¹⁄₁₀th part in linear dimensions of their parent organisms, concludes that there must be an incalculable amount of germs no larger than the ¹⁄_240,000th or ¹⁄_100,000th part of an inch in diameter; and since according to MM. Sullivant and Wormley, vision with the most powerful microscope is limited to objects of about the ¹⁄_80,000th part of an inch in diameter, we need not be surprised if Infusoria and other organisms appear in putrescible liquids in far greater numbers than the germs in atmospheric dust visible by the aid of microscopes would lead us to expect.

The ferments are the least in size and lowest in organization of all the Infusoria. The higher group which abounds in stagnant pools and ditches are exceedingly numerous, and their forms are varied beyond description. They are globular, ovoid, long and slender, short and thick, many have tails, one species is exactly like a swan with a long bending neck, but whatever the form may be, all have a mouth and gullet. Although the skin of the Infusoria is generally a mere pellicle, that of the red Paramœcium and some others resembles the cellulose covering of a vegetable cell, engraved with a pattern; but in all cases respiration is performed through the skin.

Whatever form the cell which constitutes the body of the Infusoria may have, the highly contractile diaphanous pellicle on its exterior is drawn out into minute slender cilia which are the locomotive organs of these creatures. Vibrating cilia form a circlet round the mouth of some of these animalcules, a group of very long ones are placed like whiskers on each side of it, as in the Paramœcium caudatum, and in some cases there is a bunch of bristles in front. Certain Infusoria have cilia in longitudinal rows, and in many the whole body is either partially or entirely covered with short ones. In some Infusoria their vibrations are constant, in others interrupted, and so rapid that the cilia are invisible. These delicate fibres which vary from the ¹⁄₅₀₀th to the ¹⁄_13,000th part of an inch in length, move simultaneously or consecutively in the same direction and back again, as when a fitful breeze passes over a field of corn. These animalcules seize their prey with their cilia, and swim in the infusions or stagnant pools, in which they abound, in the most varied and fantastic manner; darting like an arrow in a straight line, making curious leaps and gyrations, or fixing themselves to an object by one of their cilia and spinning round it with great velocity, while some only creep. These motions, which bring the animalcules into fresh portions of the liquid, are probably excited by the desire for food and respiration.

Fig. 104. Paramœcium caudatum. a a, contractile vesicles; b, mouth.

Fig. 105. Kerona silurus.—a, contractile vesicle; b, mouth; c c, animalcules which have been swallowed by the Kerona.

None of the Infusoria have regular jointed limbs, but certain families of the higher genera have peculiar and powerful organs of locomotion partly consisting of strong ciliary bristles placed on the anterior in rows, used for crawling or climbing, and partly consisting of groups of strong processes which serve as traction feet, generally trailing behind the animal while swimming, or used to push it forward. When the bristles or cilia of this high group of Infusoria are used for crawling their motions may be traced to the contraction of the skin; but in the Infusoria that are never fatigued though their cilia vibrate incessantly night and day, it may be presumed that these motions are altogether independent of the will of the animal, in as much as there are innumerable cilia in the human frame that are never at rest during the whole course of our existence, nor do their vibrations cease till a considerable time after death—a striking instance of unconscious and involuntary motion.

The cell which constitutes the body of the Infusoria is filled with sarcode, which is the receptacle of the food, and in that substance all the internal organs of the animalcule are imbedded. In the higher genera it is full of granular particles of different sizes and forms, and it contains a nucleus in its centre, characteristic of cellular protozoa generally. The nucleus is of a dull yellow colour, and is enclosed in a transparent capsule, which in the smaller Infusoria reflects light brilliantly. It is generally of an ovoid form and single, but in several species the nucleus is double, and in others there are several nuclei.

The Infusoria have a distinct mouth and gullet, and for the most part another aperture for ejecting the indigestible part of their food, though some discharge it by the mouth, others through any part of their surface. A few of the larger Infusoria devour the smaller; others feed on minute vegetable particles, chiefly diatoms. Solid substances that are swallowed are collected into little masses mixed with water, and enter into clear spherical spaces called vacuoles in various parts of the sarcode, where they are partially digested. When the animal has not had food for some time, clear spaces only filled with a very transparent fluid are seen, variable both in size and number. It was on account of the digestive vacuoles that the Infusoria were called Polygastria by Ehrenberg.

Transparent contractile vesicles of a totally different nature from the vacuoles are peculiarly characteristic of such Infusoria as have a digestive cavity. They exist either singly or in even numbers, from 2 to 16, according to the species, and never change their places; but they dilate and contract rhythmically at pretty regular intervals. When dilated, they are filled with a clear, colourless fluid, the product of the digestive process which they are supposed to diffuse through the body of the animal.

The Euglena, a very extensive genus of Infusoria, have smooth bodies and green particles imbedded in the sarcode, which fills their interior; and M. Wöhler discovered that the green mantle covering the saline springs at Rodenberg and Königsborner, which consists of three species of these green Infusoria, gives out bubbles of pure oxygen; thus indicating a respiratory process in these animals, the same with that in plants, namely, fixing the carbonic acid of the atmosphere and exhaling oxygen, a singularly close analogy, if not identity, of action. The Euglenæ are also distinguished by an irregular oblong space in the head filled with a red liquid; but, as it does not contain a crystalline lens, it can only be regarded as the very earliest rudiment of an eye, totally incapable of distinguishing objects, though probably sensible to the influence of light. They swim with a smooth gliding and often rotatory motion, producing a kind of flickering on the surface of the water by the lashing of a long filament attached in front, and supposed to be their only organ of locomotion; nevertheless, Mr. Gosse thinks that they are covered with most minute cilia from their manner of swimming. The Euglena acus is one of the prettiest of these little animals; it is long and slender, of a sparkling green with colourless extremities, a thread-like proboscis, and a rich crimson spot. When it swims it rotates, and a series of clear, oblong bodies are seen towards the head, and another at the tail, as if they were imbedded in the flesh round a hollow.

The Loxades bursaria, which is a giant among its fellows, has an ovoid body with green particles imbedded in its interior. The outer skin is spirally grooved, so as to form a kind of network, the elevated points of which support the cilia with which its body is beset. It has a mouth and gullet lined with cilia, which force the food in balls into the soft matter in the interior, where both the food and the green particles circulate, being carried along by a gyration of the gelatinous matter in which they are imbedded.

A species of Peridinium, which is luminous at night, and occasionally covers large portions of the Bay of Bengal with a scarlet coat by day, nearly approaches the character of the unicellular Algæ. Mr. J. H. Carter observed that at first, when these animalcules were in a state of transition, their nearly circular bodies were filled with translucent green matter, closely allied, if not identical with, chlorophyll, which disappeared when the animal approaches its fixed state, and a bright red took its place: the Infusoria were then visible to the naked eye, and the sea became scarlet. The scarlet state only lasts for a few days, for each of these innumerable Infusoria becomes encysted or capsuled, and either floats on the water, or sinks to the bottom and remains motionless. The Euglena sanguinea has a scarlet state analogous to that of the Peridium. It is so minute and versatile that it is difficult to ascertain its true form, which, however, seems to be a spindle shape, with a pointed and blunt round head. In general it is of a rich emerald green, with perfectly clear, colourless extremities; but it sometimes occurs of a deep red, and in such multitudes as to give the water the appearance of blood.[^[19]]

Fig. 106. Noctiluca.

The Noctiluca miliaris, a luminous inhabitant of the ocean, and the most beautiful of the Infusoria, is distinguished by its comparatively gigantic size, and by its brilliant light, which makes the sea shine like streams of silver in the wake of a ship in a warm summer evening, when they come to the surface in countless multitudes. It is a globular animal like a minute soap bubble, consisting of gelatinous matter, with a firmer exterior, and being about the thirtieth of an inch in diameter, it is visible to the naked eye, when a glass in which it is swimming is held to the light. On one side of the globe there is an indentation, from whence a tail of muscular fibre springs striped with transverse rings, which aids the animal in swimming. At the root of the tail lies the mouth, bordered on one side by a hard dentile lip leading into a funnel-shaped throat, from whence a long flickering cilium is protruded, supposed to be connected with respiration. The throat leads into a large cavity in the gelatinous substance, from whence the rudiments of an alimentary canal descend. From the internal surface of the globe sarcode fibres extend through the gelatinous matter, so as to divide it into a number of irregular compartments, in which vacuoles are often seen. They give buoyancy to the animal, and enable it to rise and sink in the water, but seem to disappear when the food is digested. The sarcode fibres constantly change their form and position, and the electric light emitted by a direct exertion of nerve power, which seems to be constant to the naked eye, really consists of momentary scintillations that increase in rapidity and intensity by the dash of an oar or the motion of the waves.

The Noctiluca is propagated by spontaneous division, a line appears bisecting the globe, which becomes more and more constricted till the animal is like a dumb-bell; the slender thread separating the two parts is then broken by their efforts to get free; the two new creatures swim off in different directions, and soon assume their adult form. But in many individuals there are clear, yellow globules with a well-defined nucleus, of a rich reddish-brown, which are the germs of the animal.

Most of the Infusoria multiply by continuous bisection, like the unicellular Algæ. The division generally begins with the nucleus, and is longitudinal or across, according to the form and nature of the animal, and is accomplished with such rapidity, that, by the computation of Professor Ehrenberg, 268,000,000 of individuals might be produced from one single animalcule of the species Paramœcium in a month. The Paramœcia are reproduced too by gemmation, and, as they are male and female, they are reproduced also like the higher classes.

The Infusoria have another mode of increasing. The animalcules either draw in or lose their cilia, and consequently come to rest. The animal then assumes a more globular form, and secretes a gelatinous substance from its surface, which hardens into a case or cyst, in which its body lies unattached and breaks up into minute ciliated gemmules, which swim forth like zoospores as soon as they come into the water by the thinning away of part of the cyst. In fact the animal is resolved into its offspring, which, as soon as free, gradually acquire the parent’s form, though at first they may bear no resemblance to it. The scarlet Peridium seen by Mr. Carter in the Bay of Bengal is propagated in this manner. For the parent Peridium is broken up within its cyst into from two to four new ones, each of which when set free and grown up might undergo the same process.

The Loxades bursaria increases by three distinct methods, and sometimes by two at a time. In autumn, or the beginning of winter, six or eight germs containing granular matter and one or more hyaline nuclei are formed within the animal, each enclosed in two contractile cysts: they lie freely in the cavity of the body, and come one by one into the water through a canal ending in a protuberance in the skin. During this time the pulsations of the vesicles within the Loxades are continued, but the gyration of the green particles is suspended till all the germs are excluded and swim away, and then it is renewed as vigorously as ever. At first the young are totally unlike their parent, but by degrees acquire its form. The Loxades is also increased by division, sometimes across, sometimes longitudinally, and, in the latter case, one half is occasionally seen to contain germs which have been excluded before the other half had separated, so that the two distinct systems of propagation are simultaneous.[^[20]]

The Vorticella nebulifera and some others of the Infusoria are remarkable for the diversity of their reproductive powers; for, besides division and gemmation, they are reproduced by a kind of alternate generation, accompanied by singular metamorphoses. The Vorticella, one of the most beautiful animals of its class, lives in pools of fresh water: groups of them are found on almost every mass of duckweed like little blue bells upon slender stalks, creating active currents in the water by the vibrations of long and powerful cilia with which the margin of the bell is fringed. The lip or edge of the bell is bent outwards into a permanent rim, and a deep groove cleaves the rim on one side, in which a wide cavity forming the mouth is placed. The mouth, the short throat or gullet, and the whole bell, are bristled with vibratile cilia.

Fig. 107. Vorticellæ.

The Vorticellæ feed on vegetable organisms, chiefly diatoms, and are exceedingly voracious. The cilia round the rim of the bell entangle the food, draw it into the mouth, and those in the gullet force particle after particle mixed with water into vacuoles which they make in the interior of the soft sarcode which fills the bell, and there the particles undergo rotation till digested and absorbed, and, if refuse remain, it is ejected through a softer part in the outer layer of the bell.

Fig. 108. Acineta.

The stem that fixes the animal to a solid object is a tubular continuation of its outer membrane, containing a highly contractile filament; and, as the creature is extremely sensitive to external impressions, it folds up the ciliated rim of its bell, and its stalk shrinks down in a spiral on the slightest alarm, but the bell opens and the stalk stretches out again as soon as the alarm is over. When a Vorticella is reproduced by division, the bell separates longitudinally into two parts; one is often smaller than the other, and separates from its parent, swims about till it gets a stem, and fixes itself to an object. When the two parts are of equal size, the division extends to a greater or less distance down the stalk, and as each of these become perfect bells, and do not fall off but subdivide in the same manner, it follows that, by successive divisions, a whole group of these beautiful animals may spring from the same stem, as in [fig. 107].

The Vorticella has a most wonderful mode of reproduction common to a few other Infusoria. A gelatinous substance is secreted by the bell, which hardens and envelopes it in a cyst; the encysted bell then separates from its stalk, and is transformed into an infusorial animal called an Acineta ([fig. 108]), closely resembling the Actinophrys sol with radiating filaments which it continually stretches out and draws in. A motile ciliated embryo, or Vorticella bud, is then formed within the Acineta, which, after a time, comes out at a slit in its side, swims about, gets a stem, fixes to some object, and is developed into a Vorticella. The slit closes again, and the Acineta keeps moving its filaments as usual, and another motile embryo is formed within it, which is emitted by a slit in the same manner, and is also developed into a Vorticella. As these young Vorticellæ, or bell animals, may undergo the same transformations, there may be an indefinite alternation of the two forms. The Vorticella-bud, when it issues from the slit in the Acineta, has an oval form, with a circlet of long cilia at its narrow end, a mouth at the more obtuse, a nucleus, and contractile vesicles, and, after swimming about till it finds a suitable place, it becomes fixed by one end of its oval body, a style or stem is formed, which rises rapidly, and the adult shape is developed. The Acinetæ are said to live upon Infusoria: they apply the dilated apex of their rays as sucking discs to the animal, and suck its contents till it dies. The Tricoda linceus undergoes metamorphoses analogous to those of the Vorticella, but more numerous and complicated.[^[21]]

Most of the Vorticellæ, and probably the majority of Infusoria, remain unchanged for a time within their cysts, being then in a state analogous to the hybernal sleep of some of the reptiles. The cyst shelters them from cold and draught, and, when heat and moisture are restored, they resume their active vitality. The motions of the Infusoria are probably automatic, and in some instances consensual; they have neither true eyespecks, though their whole body seems to be conscious of light and darkness; nor have they ears; and, with the exception of touch, which the Vorticellæ have in a marvellous degree, it may be doubted whether the Infusoria have any organs of sense whatever, though they avoid obstacles and never jostle one another. The vibrations of their cilia are involuntary as in plants, an instance of the many analogies which perpetually occur between the lowest tribes of the two great kingdoms of nature. In both there are examples of propagation by bisection, conjugation, budding, and the alternation of generation, which occurs more frequently among Protozoa than among any other class of animals. There is a perfect resemblance between Zoospores and Protozoa; they both cease to move, the Zoospore when it secretes its cellulose coat and becomes a winter or resting spore, the Protozoon previous to encysting, a process presumed to be universal among that class of animals, before subdivision or reproduction begins. It is the dried cysts or germs of the Infusoria that float in the atmosphere as winter spores do, and it is believed that, like the fungi, the same germs may develope themselves into several different forms according to the nature of the liquid into which they may chance to be deposited; consequently, it is not necessary that the variety of germs should be very great, although the Infusoria themselves are of numerous forms.[^[22]]

The Infusoria, the smallest of beings, apparently so insignificant, and for the most part invisible to the unaided eye, have high functions assigned to them in the economy of nature. They ‘are useful for devouring and assimilating the particles of decaying animal and vegetable matter from their incredible numbers, universal distribution, and insatiable voracity—they are the invisible scavengers for the salubrity of the atmosphere. They perform a still more important office in preventing the gradual diminution of the present amount of organic matter upon the earth. For, when this matter is dissolved or suspended in water in that state of comminution and decay, which immediately precedes its final decomposition into the elementary gases, and its consequent return from the organic to the inorganic world, these wakeful members of Nature’s invisible police are everywhere ready to arrest the fugitive organic particles, and turn them back into an ascending stream of animal life. Having converted the dead and decomposing matter into their own living tissues, they themselves become the food of larger Infusoria, as the Rotifera and numerous other small animals, which, in their turn, are devoured by larger animals as fishes, and thus a pabulum fit for the nourishment of the highest organized beings is brought back by a short route from the extremity of the realms of organized matter.’[^[23]]

SECTION III.
HYDROZOA, ZOOPHYTES.

Zoophytes are animals of a much higher organization than the Protozoa, inasmuch as they are furnished with special organs of prehension, offence and defence, of attachment, and in many of locomotion. For the most part they consist of numerous individuals called Polypes, united in a community, and living together in intimate sympathy and combined action, so as to form one single compound animal.

Zoophytes are divided into two groups, namely the Hydrozoa, whose type is the common fresh-water Hydra, and the Actinozoa, which are composite animals, including the reef-building corals, whose polypes are formed according to the type of the Actinia, or common Sea Anemone. The Hydrozoa consist of seven orders, the first of which are the Hydridæ, inhabitants of fresh water; the next constitute the oceanic Hydrozoa, some of which, though extremely varied in form, are connected by the most wonderful relations.

The solitary Hydra that lives in fresh-water pools and ditches, consists of a soft cylindrical muscular bag, capable of being stretched into a slender tube, shrunk into a minute globe, or widely distended at will. At one end there is a circular mouth, which is highly sensitive, opening, closing, or protruding like a cone, and surrounded at its base by six long flexible arms called tentacles, arranged symmetrically. The mouth opens into a cavity extending throughout the length of the body, which is the stomach; the other end of the sac is narrow, and terminates in a disk-shaped sucker, by which the Hydra fixes itself to aquatic plants, or floating objects, from whence it hangs down, and the tentacles float in the water.

Fig. 109. Thread-cells and darts.—A, B, C, D, Thread-cells at rest; E, F, G, H, appearance of the darts when projected.

The sac or body is formed of two layers, an inner and an outer layer, of firmer texture, formed of cells imbedded in a kind of sarcode, and the space between the two layers is filled with a semifluid substance, mixed with solid particles and full of vacuoles. The inner and outer layers are united at the mouth, and the tentacles are closed tubes in communication with the cavity of the stomach. The exterior layer of the tentacles is beset with wart-like excrescences, formed of clusters of cells, with a larger one in the centre filled with a liquid. In all of them a long spicula, or sting, often serrated at the edge, is coiled up like a thread, and fixed by one end to a kind of tube, like the inverted finger of a glove, that the animal can dart out in an instant.

Thus armed, the tentacles are formidable weapons; they are highly contractile and wonderfully strong, tenaciously adhering to the small worms and aquatic insects on which the Hydræ feed, and they are aided by the roughness of their surface. They transfix their prey, and are believed to infuse a liquid poison from the dart, or thread-cells, into the wound, then twisting their other tentacles round the victim, it is instantly conveyed to the mouth, and slowly forced into the digesting cavity, where it is seen through the transparent skin to move for a short time, but as soon as the nutritious juice is extracted, the animal ejects the refuse by its mouth. In the inner layer, enclosing the cavity of the stomach, there are cells containing a clear liquid with coloured particles floating in it, which is supposed to perform the part of a liver; and, as the Hydræ have no respiratory organs, their juices are aërated through their skin. They have no perceptible nerves nor nerve centres, yet they are irritable, eminently contractile, and are attracted towards the light—all these being probably sympathetic motions.

Though in general stationary, the Hydra can change its place; it bends its body, stretches to a little distance, and fixes its anterior extremity firmly by its tentacles; then it detaches its sucker and brings it close to its mouth, fixes it, and again stretches its fore part to a little distance along its path, and repeats the same process, so that it moves exactly after the manner of certain caterpillars. It can even move along the water by attaching the expanded disk of its sucker to the surface, where it soon dries on being exposed to the air, and becomes a float, from whence the Hydra hangs down with its tentacles extended like fishing lines, as in [fig. 110]; or it can use them as oars to row itself along under the surface of the water.

On account of their simple organization, the Hydræ are endowed with the most astonishing tenacity of life. As the whole animal is nourished from the surface of the digestive cavity, they appear to suffer no inconvenience from being turned inside-out, the new cavity performing all the functions of digestion as well as the old one. They may be cut into any number of pieces, and, after a little time, each piece becomes a perfect Hydra. The head may be cut off and they get a new one; or it may be split into two or three parts or more, and the animal becomes many-headed; and, what is still more marvellous, two Hydræ may be grafted together direct, or head and tail, and they combine into one animal.

Fig. 110. Hydra fusca.

These singular and voracious creatures increase like plants by budding. A little protuberance rises on the body by the bulging out of the double skin or wall, so that the interior of the bud is a clear cavity in communication with the stomach of the Hydra ([fig. 110], b). The bud increases in length, opens at its extremity into a mouth, and gradually acquires the size and form of its parent ([fig. 110], c); the communication is then by degrees closed, and at last the matured bud drops off and becomes an independent Hydra. Dr. Carpenter observed that this process, which so closely resembles the budding of plants, must be regarded as a modification of the ordinary nutritious process. The same may be said of the power of reparation, which every animal body possesses in a greater or less degree, but which is most remarkable among the lower tribes, for when an entire member is renewed, or even when the whole body is regenerated from a small fragment, which is the case in many polypes, it is by a process exactly analogous to that which takes place in the reparation of the simplest wound in our own bodies, and which is but a modification of the process that is constantly renewing, more or less rapidly, every portion of our frame.

There is but one species of the single colourless Hydra, but there are four compound fresh-water Hydræ in England—the rubra, viridis, vulgaris, which is of an orange brown, and the fusca. They have coloured particles, either imbedded in their external coat, or immediately under it. The Hydra viridis and H. vulgaris have short tentacles, whilst H. fusca, which is a rare animal, has arms from seven to eight inches long, and so contractile, that they can shrink into the space of small tubercules. All these four Hydræ are compound and permanently arborescent animals; each springs from one individual hydra of its own race, which increases in length and forms the stem, while young ones spring from it and from one another consecutively, like the compound branches of a tree. The numerous tentacles that hang down like fishing lines, thickly covered with thread-cells and their envenomed darts, catch prey for the whole colony, because the communication between the stomachs of the young polypes or Hydræ and that of their parent is never cut off, as it is when the offspring is deciduous; but tubes from the base of each individual Hydra or polype, passing through the stalks and branches of the living tree, unite their stomachs with the stomach or assimilating cavity in the main stem. Each individual polype, sometimes to the number of nineteen, after having digested its food or prey, ejects the refuse from its mouth, and the nutritious juice traverses the labyrinth of tubes to that general reservoir.

Since every portion of the bodies of the Hydræ is nearly of the same kind, and as every part of their surface inside and outside is in contact with the water in which they live, and from whence they derive oxygen to aërate their juices, no circulation is necessary in these simple animals, either for nutrition of their tissues, or to furnish them with oxygen.

If the Hydræ only produced deciduous buds which are developed into facsimiles of their parent, their race would become extinct, since they die in winter, unless kept artificially in water of mild temperature; but the animals are hermaphrodite, so that each individual produces fertilized eggs in autumn, which are hatched in spring, so that the Hydra is alternately propagated by deciduous buds and by eggs. The fresh-water hydræ are the only hydroids that are locomotive, all the others being fixed to some solid substance.

The oceanic Hydrozoa comprehend the three families of Corynidæ, Tubulariidæ, and Sertulariidæ. They are chiefly compound animals, numerous in genera and species, and have great variety of form. They may be simple and slender, they may be creeping or like a bush or tree, more or less compound and regularly branched according to the form of the polypary or tubular substance which unites their numerous hydra-form polypes into one animal. In general they are exceedingly small; three or four inches in height is quite gigantic. There is scarcely a still clear pool left by the retiring tide among the rocks along the British coasts, that does not abound with these beautiful creatures attached to stones, old shells, or sea-weeds. But they must be sought for amidst the luxuriant marine vegetation and profusion of animal life which adorn these rocky pools, otherwise they would escape notice; and even when large enough to be conspicuous, the eye must be aided in order to see the wonderful minuteness and delicacy of their structure. The aquaria have furnished an opportunity to study their forms, habits, and the marvellous circumstances of their lives and reproduction.

The compound oceanic Hydrozoa are essentially the same in structure as the compound fresh-water Hydræ. They differ, however, from them in often having a greater number of tentacles, and in being defended by a firm and flexible horny coat; notwithstanding which they increase in size by budding from the base of a single primary polype. The horny coat covers the bud and grows with it; but as soon as the polype is formed within it, the top of the bud opens and the young polype protrudes itself, so that a separation is effectually prevented; and while the stem and branches are being formed, and increase by the continual development of new buds, the communication between the stomachs of the whole brood of polypes with that in the parent stem is maintained by tubes from their bases passing through the interior fleshy matter in the branches.

In short these marine Hydrozoa consist of a ramified tube of sensitive animal matter, covered by an external flexible and often jointed and horny coat or skeleton, and they are fed by the activity of the tentacles and the digestive powers of frequently some hundreds of hydra-formed polypes, as in the Sertularia cupressina. The common produce of their food circulates as a fluid through the tubular cavities, for the benefit of the whole community, while the indigestible part is ejected from the mouth of each individual. The stomach of each polype has a more or less ciliated lining, containing cells with nutritive juices, which are supposed to perform the part of a liver. The liquid which circulates in these animals is colourless, with solid particles floating in it; and there is reason to believe that sea-water is admitted into the tubes, and that, mixed with the juices prepared by the polypes, it circulates through the ramified cavities, is sent into the hollow prehensile tentacles, and returns back into the digesting cavity after having contributed to respiration by its oxygen. The movements of this fluid appear to depend upon the delicate ciliated fibre which lines the cavities of the tentacles and those of the stem and branches of the compound animal, possibly aided by vital contraction. The soft skin of the tentacles contains cells full of liquid, with a thread and its sting or dart coiled up within it. These thread stings are protruded when the skin is irritated, which frequently gives the tentacles the appearance of being beset with bristled warts. In many instances these kinds of Hydrozoa are covered with a gelatinous substance, either as a film or thick coat.

The reproduction of many of these arborescent or compound Hydrozoa is one of the most unexpected and extraordinary phenomena in the life-history of the animal creation. For besides the system of consecutive budding from a single polype which builds up the compound animal, peculiar buds are formed and developed, which bear no resemblance whatever to the polype buds: on the contrary, when mature, they assume an organization exactly the same as that of the common jelly-fish or Medusoid Acalephæ, and swim freely away from their fixed parent as soon as they are detached. These medusiform zooids, which are extremely small, consist of a cup or umbrella-shaped bell of colourless transparent matter, which is their swimming apparatus; it is contracted and expanded by a muscular band under the rim, the water is alternately imbibed and forcibly ejected, and by its reaction the zooid is impelled in a contrary direction. From the centre of the bell a stomach hangs down in the form of a proboscis, with a mouth at its extremity, either with or without tentacles and sting-cells. Four canals, or a greater number, which begin in the stomach, radiate through the transparent matter of the bell, and are united by a circular canal round the rim; they convey the nutritious liquid from the stomach throughout the system. This general structure may be traced in the zooids of the three great families of the oceanic hydraform-zoophytes, in a greater or less degree, from deciduous perfect medusæ to such as are imperfect and fixed.

These medusiform zooids are male and female, and when detached from their parent they are independent creatures, each of them being furnished with nutrient and locomotive organs of its own. They produce fertilized eggs, which are developed into ciliated locomotive larvæ; after a time these lose their cilia and acquire a rayed sucking disc, with which they fix themselves permanently to a solid object, and, after various changes, each gets a mouth and tentacles and becomes a perfect young hydra. Thus a brood of young hydræ is produced, each of which acquires the compound form of its parent by budding, and as each of these compound animals in its turn gives off medusa-buds, there is a cycle of the alternate forms of hydra and medusa or jelly-fish, showing a singular connection between two animals which seem to have nothing in common. The analogy which so often prevails between plants and animals obtains here also, for the medusa-buds bear the same relation to the hydra or polype-buds that the flower-buds of a tree do to the leaf-buds: the flower-buds contain the germs of future generations of the tree, while the leaf-buds contain only the undeveloped stems, stalks, and leaves of the individual plant on which they grow.

The Corynidæ form the first of the three families of the oceanic hydra zoophytes. They comprise six genera, and many species of compound animals of various forms, each derived from a single animal by budding; and although they possess a thin flexible coat, the polypes are sheathed either in a thin membrane or bone. Their club-shaped tentacles form either a single or double circlet round the base of their conical mouth, and are also scattered over their bodies when bare.

Fig. 111. Syncoryna Sarsii with Medusa-buds.

The zooids are developed at once in the Syncoryna Sarsii, which is a long, thinly branched, and horny zoophyte, with a single naked, spindle-shaped polype at the extremity of each branch, as in [fig. 111], A. The bodies of the polypes are studded with numerous tentacles, among which buds appear ([fig. 111], a, b); these gradually expand into bell-shaped medusa-zooids ([fig. 111], c), some being masculine and others feminine. They drop off their parent, swim away by the contraction of their bell, and their fertilized eggs are developed into single hydræ, which become arborescent like their parent by budding.

The family of the Sertulariidæ take branching forms, sometimes of perfect symmetry: they have a firm, horny coat, which not only covers the stem and branches, but becomes a cup for the protection of the polype. The most common form of the family of the Tubularia has no branches: it has an erect, hollow stem like a straw, sometimes a foot high, coated by a horny sheath. The polype which terminates each plant has a mouth surrounded by alternately long and short tentacles. The stomach of the polype is connected with the hollow in the stem by a muscular ring, by whose alternate dilatation and contraction, at intervals of eighty seconds, the fluid is forced up from below, enters the stomach, and is again expelled. Another liquid carrying solid particles circulates in a spiral through the whole length of the stem. Some of this family are propagated by perfect deciduous medusæ, others by imperfect fixed ones; both are developed on the polypes or among their tentacles. Like the fresh-water Hydræ, these creatures can restore any part of their bodies that is injured.

Numerous instances might be given to show that the minute medusiform zooids are only a stage or phase in the life of an oceanic hydra: conversely it will now be shown, that the single simple hydra is but a stage in the life-history of the highly organized medusa, jelly-fish, or sea-nettle of sailors, the Acalepha of Cuvier.

The medusæ vary in size, from microscopic specks that swim on the surface of the sea in a warm summer day to large umbrella-shaped jelly fish almost a yard in diameter. They abound in every part of the ocean and in all seas, often in such shoals that the surface of the water is like a sheet of jelly. Their substance is transparent, pure, and nearly colourless; chiefly consisting of water, with so little solid matter, that a newly caught medusa, weighing two pounds, dries into a film scarcely weighing thirty grains.

The Pulmograde Medusæ, which swim by the contractions of their umbrella-shaped respiratory disc, form two distinct groups, the naked-eyed medusæ and the covered-eyed group. Both are male and female; each has its own form of thread-cells; and the stinging power or strength of the poison is nearly in proportion to the size of the animal and the coarseness of its threads.

The disk, or umbrella-shaped swimming organ, in both groups consists of a large cavity included between two layers of gelatinous matter, which unite at the rim. The interior membrane, called the sub-umbrella, is encircled at its edge by a ring of highly contractile muscular fibre like the iris of our eyes, by which this swimming organ is expanded and contracted. From the centre of the sub-umbrella a stomach, in the form of a proboscis, is suspended, which is of a very different structure in the two groups.

Fig. 112. Thaumantia pilosella.

The Thaumantia pilosella, a member of the naked-eyed group, is like an inverted watch-glass ([fig. 112]), less than an inch in diameter. The roof of this umbrella is much thicker than the sides, and gradually thins off towards the rim. The proboscis, or stomach, descends from the centre of the sub-umbrella, but not so far as to the edge of the rim: it ends in a mouth with four sensitive fleshy lips. Four slender canals, which originate in the cavity of the stomach, radiate from the centre of the roof of the umbrella and extend to its margin, where they unite at the quadrants with a canal which encircles the rim, and are prolonged beyond it in the form of tentacles armed with numerous thread-cells containing poisonous darts. These tentacles must be formed of muscular fibre, for they are very irritable: each of them may be extended and contracted separately or along with the others; they guide the medusa through the water, and can anchor it by twisting round a fixed object.

The prey caught is digested in the stomach, the refuse is ejected by the mouth, and the nutritious fluid that has been extracted is carried up through the base of the stomach into the four radiating canals, to supply the waste and nourish the system. The digestive cavity and canals are lined with a soft membrane, covered with cilia, whose vibrations maintain the circulation of the juices and perform the duty of a heart; for the medusæ have none, nor have they any special respiratory system: their juices are aërated through the under-surface of the rim of the umbrella, while passing through the circular canal lying either within the water or on its surface.

A fringe of filamental tentacles hangs down into the water from the rim of the disc or umbrella, which is studded at equal distances by fleshy bulbs, each of which has a group of fifty dark eye-specks, being the rudiment of an eye; and if the animal be disturbed when in the dark, each eye-speck shines with a brilliant phosphoric light, and the umbrella looks as if it were begirt with a garland of stars.

Fig. 113. Otolites of Magnified Thaumantias.

Close to the edge of the canal which encircles the margin of the umbrella, there are eight hollow semi-oval enlargements of the flesh, two in each quadrant formed by the four radiating canals: they are the eight ears of the medusa, for in these hollow organs there are from thirty to fifty solid, transparent, and highly refractive spheres, arranged in a double row, so as to form a crescent, those near its centre being larger than the more remote. The solid spheres are analogous to the otolites in the ears of the more highly organized animals. Mr. M‘Cready has discovered nerve-centres behind each tentacle, and under each marginal coloured speck in several species of the open-eyed medusæ, which places this group of Acalephæ in a higher grade than any of the preceding orders. The medusæ swim by the muscular energy of their umbrellas: at each rhythmical contraction the water, which enters by the mouth and fills the great central cavity within the umbrella, is forced out again through an orifice at the other end, and by its reaction the medusa is impelled with considerable velocity in the contrary direction, so that the top of the umbrella goes first, and all its tentacles are dragged after it.

The medusæ are diœcious: in the males four reproductive cells full of reddish or purple granular matter surround the cavity of the stomach, and appear like a coloured cross through the top of the gelatinous umbrella. In the females, at a point just before the four radiating canals enter the marginal canal, the flesh on the exterior of the umbrella swells out into bulbs, containing vessels full of clear eggs with minute globular yolks. These eggs, when fertilized, are hatched, and the young are developed within these ovaries, so that they come into the water as a kind of infusorial ciliated animalcule destitute of a mouth. One end of the creature acquires a suctorial disc, fixes itself to an object, and uses its cilia. The other end opens into a mouth, round which tentacles like fishing lines spring forth; the central part is converted into the cavity of the stomach, and thus a perfect hydra is formed, capable of being propagated naturally by budding, or artificially by being cut in pieces, each piece becoming a perfect hydra, differing in no respect from a common simple fresh-water Hydra.

Fig. 114. A, B, C, D, development of Medusa-buds; a, polype-body; b, tentacles; c, a secondary circle of tentacles; d, proboscis; e, new polype-bud.

From one of these, numberless successive generations of simple hydræ may be produced by budding, all catching their prey with their tentacles and digesting it in their stomachs. The limits to this budding-system seems to be indefinite: years may pass in this stage, but at length it ceases, and either the original hydra, or one of its descendants, undergoes a series of remarkable changes. The body of the hydra lengthens into a cylinder; it is then marked transversely by a number of constrictions beginning at the free end; these become deeper and deeper, till at length they break up the body into a pile of shallow cups, each lying in the hollow of the other, and leaving a kind of fleshy wall at the point of suspension or fixture. The edges of the cups are divided into lobes with a slit in each, in which the coloured rudiment of the eye is sunk. The cups are permanent, and characteristic of the group of naked-eyed medusæ. After a time, the cups begin to show contractile motions, which increase till the fibre of their attachment is broken, and then the superimposed cups are detached from the pile one after another, and swim freely away by the contractions of their lobes as young medusæ, leaving what remains of the parent hydra to repair its loss and again repeat this singular process. However, the young medusæ are not yet perfect. As they increase in size the divisions on the edge of the cup fill up; a proboscis-shaped stomach, with its four coloured cells and its square mouth, is developed from the centre of the sub-umbrella; the radiating canals extend from the central cavity, the encircling canal and fringe form round the umbrella-shaped cups, and the result is a highly organized Thaumantia pilosella, in whose life-history a simple hydra forms a singular stage.

Thus hydræ produce medusæ whose offspring are hydræ, and perfect medusæ produce hydræ whose offspring are perfect medusæ. However, the law of the alternation of generation is by no means peculiar to the Thaumantiæ. Many species of medusæ are subject to it, as the Turris neglecta, a beautiful little medusa not larger than a hempseed, common on the British coasts. It has a white muscular pellucid umbrella, a large proboscis of a rich orange colour at its upper part: in the orange-coloured flesh of it there are ovaries containing rose-coloured eggs, which are hatched within them, and come into the water as ciliated gemmules, which, after swimming about for a time, become fixed and are developed into small hydræ of a rich purple colour with sixty-four tentacles. From these hydræ others bud off indefinitely till the time comes when one of them becomes lengthened, constricted, divided into cups which drop off, and finally become a brood of the Turris neglecta.

The naked-eyed medusæ are extremely numerous. There are six orders of them and many genera, chiefly distinguished by the position and nature of their ovaries and the number of canals which radiate through their swimming organs. Both of the medusæ that have been described have four radiating canals; yet they belong to different orders, for the ovaries of the Thaumantia are in the edge of the umbrella, while those of the Turris are in the substance of the proboscis. Neither of these kinds have more than four ovaries, but some other kinds have eight ovaries and eight radiating canals. Most of the canals are simple, but in one genus they are branching. All are furnished with tentacles, some of them having stings, others none.

The covered-eyed group consists only of two natural divisions—the Rhizostoma, or many-mouthed medusæ, and the Monostoma, or one-mouthed medusæ. In both the coloured eye-specks at the margin of the umbrella are larger and more numerous, than in the naked-eyed group, and they are covered with a hood. The proboscis of the one-mouthed order terminates in a square mouth, the four angles of which are prolonged into tentacles with a solid hyaline axis. They have a fringed membrane along their under-surface, containing numerous stinging thread-cells. Sixteen canals, connected with the stomach or cavity of the proboscis, radiate over the flattish, cup-shaped umbrella; eight of these are branched, and terminate in the circular canal which runs round its fringed edge, and they form the nutrient and respiratory system of the animal, while the eight simple and alternate canals terminate in eight openings at the rim of the umbrella, through which the refuse or indigestible part of the food is discharged, thus forming an exception to the other pulmograde medusæ, and indeed to the Hydrozoa in general, which eject it at the mouth. All the canals are lined with cilia, whose vibrations maintain the circulation of the fluids, and perform the duties both of a heart and respiratory apparatus. Dr. A. Krohn has observed that in three species of the genus Pelagia belonging to the covered-eyed medusæ, the young are at once developed as medusæ without the intervention of the hydra form.

Fig. 115. Rhizostoma.

The disk of the Rhizostoma, or root-mouthed medusæ, is rather flat, and the large proboscis is unlike any other of the tribe. In the naked-eyed medusæ digestion is performed in the cavity of the proboscis; but in this order the proboscis is divided into four very long branches ending in club-shaped knobs ([fig. 115]), and nutrient tubes extend to their extremities from the great central cavity in the umbrella. Their broadish frilled borders are divided and subdivided along their whole lengths, and the nutrient canals, which follow all their ramifications, end in numerous fringed pores upon their edges and upon the club-shaped ends of the quadrifid proboscis. These numerous pores are mouths; they absorb minute animalcules, which are digested while passing through the united canals to the great central cavity of the umbrella, which receives the products of digestion. Eight canals radiate from that great cavity and traverse the umbrella; and the nutrient fluid, mixed with the sea-water, passes from the great cavity through these canals into an elegant network of large capillary tubes spread on the under-surface of the margin of the umbrella, which is always in contact with the water; and in this beautiful respiratory organ the carbonic acid gas is exchanged for the oxygen in the water of the sea. The indigestible part of the food is discharged through the mouths or pores, whose edges are prolonged into solid tentacles containing thread-cells, with their usual weapons of offence and defence. Besides these armed tentacles, which are very numerous in the covered-eyed group, the gelatinous umbrella has a multitude of oval thread-cells on its external coat, in each of which a very long filament is spirally coiled, which darts out to a considerable distance on the smallest touch, and stings severely.

A few only of the British pulmonigrade medusæ sting: the Cyanea capillata, one of the single-mouthed covered-eyed family, is most formidable. It has very long tentacles, which it can throw off if they get entangled, but they continue to sting, even after they are detached from the medusa.

This is one of the most remarkable instances of the inherent irritability of muscular fibre still in full force after the tentacles have been separated from the living animal. In many of the lower animals, as in the Hydra itself, vitality is so far from being extinguished in the severed members that it repairs the injury. Since the covered-eyed medusæ have eyes, ears, and very sensitive tentacles, it may be inferred that they possess nerves of sight, hearing, and touch, though none have been discovered, probably on account of the softness and transparency of their tissues. The stinging power by which they kill their prey and defend themselves may be classed among the consensual powers prompted by the sympathetic sensations of hunger or danger.