Natural History, Mollusca/Nudibranchiata
ORDER NUDIBRANCHIATA.
(Naked-gilled Mollusks.)
Scarcely any of the animals which the marine naturalist meets with in his dredging voyages, or his sea-side excursions, are so attractive as those of this Order. They are remarkably elegant in their forms, which present great variety. Their motions are graceful and lively, their colours peculiarly brilliant, and their history and economy marked by points of great interest. Nor is it rarely that the zoologist is gratified with an opportunity of observing these very attractive Mollusks, for the species are numerous, the British shores alone yielding more than a hundred.
These are not air-breathers, like the members of the preceding Order; they are all marine, and respire by means of gills, which are not enclosed within the body, but (at least during action) exposed freely to the surrounding medium. Sometimes these organs are capable of being partially withdrawn into a cavity, situated on the medial line of the back; but more commonly they take the form of simple or branched warts, arranged along the sides. The foot is large and broad. The mantle is ample, and projects above the foot on all sides. None of the species are protected by a shell, except in early infancy. The young, on emerging from the egg, is enclosed in a shell, which closely resembles that of a nautilus, and bears very little resemblance to the parent in its form; it has, therefore, to pass through a metamorphosis before it attains its permanent condition.
LARVA OF EOLIS.
The eggs in all the species are numerous. They are deposited, during the spring and summer, commonly in the form of a broad ribbon of clear jelly, attached by one of its edges to some solid substance, and generally coiled, or irregularly twisted, or frilled. The eggs themselves are arranged in close-set rows, crossing the gelatinous belt, and giving an opaque white appearance to the mass, which would otherwise be colourless.
In general each egg-shell (chorion) contains but a single yolk, but in some of the Dorides each contains two or three; and in the elegant Antiopa cristata, a specimen of which lately spawned in my possession, I found, upon the average, the extraordinary number of sixty yolks in each egg shortly after deposition. The yolk, which is contained within a delicate, transparent, membranous shell or chorion, may be observed changing form. The time required to mature the embryo varies in different species, and probably in the same species under different circumstances; in general from ten to fifteen days is sufficient for this purpose.
The development of the embryo is highly interesting, especially as, owing to the transparency of all the organs, nothing is easier than to watch its progress. I have kept many specimens in glass vases of sea water; and as all the species of this Order are hermaphrodite—that is, the sexual functions are united in the same individual—every specimen obtained is pretty sure to spawn during the season. The yolk, which at first nearly fills the egg-shell, soon becomes a little elongated, with one end diagonally truncated, or, as it were, cut off obliquely; the truncated end then becomes two-lobed, "each lobe exhibiting an imperfect spiral, and having its margin ciliated. The now animated being is seen to rotate within its prison. Shortly the lobes enlarge, and a fleshy process, the rudimentary foot, is observed to develop itself a little behind them, on the medial line; a shell closely investing the inferior portion of the embryo, the lobes and rudimentary foot being uppermost. The shell rapidly increases, and assumes a nautiloid form; afterwards the foot displays, attached to its posterior surface, a circular operculum, which is opposed to the mouth of the shell. The lobes now expand into two large, flattened, ovate appendages, with very long vibratile cilia around the margins, and the larvæ are at length mature. The whole mass of spawn now presents the utmost animation. Hundreds of these busy atoms are seen, each within its transparent, membranous cell, rotating with great agility and ceaseless perseverance, the cilia all the while vigorously vibrating on the margins of the outstretched lobes. The membranous chorion, which by this time has become enlarged, ultimately gives way, no longer able to resist the perpetual struggle within; and the liberated larva, wending its way through the shattered shreds of the general envelope, boldly trusts itself to the open trackless water, where, doubtless, thousands and tens of thousands perish ere they find a fitting resting-place, some being swept away by resistless currents, others falling a prey to ever-watchful and innumerable enemies."[1]
As the larva increases in age, the foot acquires considerable dimensions. The operculum becomes delicate and transparent, so as to be with difficulty examined; it is, however, seen to be circular, and concentric lines seem to indicate that its growth takes place in the ordinary manner. Besides the long cilia of the lobes, the action of which is under the control of the larva, "the whole surface of the exposed parts is covered with minute cilia, whose vibrations never cease. These cilia serve most probably for respiratory purposes, and may also assist in bringing food to the mouth.
"When the larva is at rest, the oral lobes are pulled back into the shell, and the foot being drawn down, brings along with it the operculum, which closes the orifice. But when in action, the whole of these parts project beyond the opening of the shell, the foot lying back against the spire; and the oral lobes inclining forward, their cilia commence to vibrate, and the larva, with the mouth of the shell upwards, moves through the water with lively action, sinking, or rising, or advancing onwards at its pleasure."
Messrs. Alder and Hancock, from whose beautiful "Monograph of the British Nudibranchiate Mollusca" I cite these particulars, "have not succeeded in tracing the development of the larva into the mature form; but it is not difficult to understand how this change is effected. When the larva is placed with the mouth of the shell downwards, the oral lobes in front, (speaking particularly of Eolis,) the anal termination of the intestine, and the oval sac representing the generative organs, will be found on the right side, close to the base of the oral lobe, and the operculigerous lobe or foot will be seen to extend backwards in a median position, occupying the place of the crawling disk. Thus it is evident that the principal organs of the larva only require to be slightly modified in form, and it is changed into the mature animal, the shell and operculum being cast off, and the oral lobes either absorbed or altered into a veil or oral tentacles."
The food of the Nudibranch Mollusca is for the most part animal. Various kinds of zoophytes are devoured by many species, some are even cannibals, preying upon their own kind; but some of the Dorides appear certainly to be herbivorous, feeding on the fronds of sea-weeds. Professor Grant has often found their stomachs completely filled with minutely divided portions of coarse marine plants, filling not only that organ, but also the cavities of the liver, which in these animals is very large. The mouth and its various parts are very efficiently adapted for the seizing and devouring of food. Various parts are armed with points and cutting edges in different genera; but the tongue is, I believe, in all most elaborately armed with spinous teeth, arranged in numerous rows.
TONGUE-TEETH OF EOLIS.
(Magnified.)
The mouth has large fleshy lips prolonged into an internal tube, within which is a muscular inner lip, capable of being pushed forwards, and bringing with it the mouth and jaws when the animal is in the act of seizing its prey. The mouth contains two large horny jaws placed vertically, and armed with cutting edges. The tongue is shaped like a strap, and can be moved in different directions, or thrown forwards to assist the animal in seizing and securing its prey. It is covered with numerous transverse plates, armed with spines or teeth directed backwards. These teeth differ in number and in form; those of Eolis papillosa, figured above, are very numerous and minute, being not more than one-sixth part of the thickness of a human hair. Their arrangement is in transverse arched rows, but in E. coronata, there is one large central tooth on each plate with denticulated sides; and in E. alba, a central tooth only, without denticulations.
Family Dorididæ.
The members of this extensive family are characterised by having the anal orifice placed in the medial line of the back, towards the hinder part, and the respiratory organs arranged around it in a more or less complete circle of leaf-like plumes, so as to resemble the petals of a beautiful flower. The mouth is a small conical proboscis placed under the front edge of the mantle, where this latter organ is distinct, and sometimes guarded by two simple tentacles; another pair of tentacles is placed on the front part of the back. These are peculiar in their structure, being beset with numerous narrow plates, arranged diagonally, parallel to each other. These tentacles, for the most part, are capable of being withdrawn into proper cavities, the edges of which, in many cases, are raised into tubular sheaths to protect them.
The Dorididæ are found on the shores of every sea. Many species are common on our own coasts, crawling on sea-weeds, or concealed beneath stones between tide-marks; some kinds, however, confine themselves to deep water, and are to be obtained only by dredging.
Genus Doris.
The body in this, the principal genus, is usually of a semi-oval form sometimes very convex, but more generally depressed, and occasionally almost flat. The mantle is ample, projecting over the head, and on all sides; its surface is generally rough, with numerous warts, and its texture is stiffened with calcareous spicula or crystals of lime imbedded in its substance. The mouth is commonly without jaws, but is frequently furnished with a prehensile collar, armed with minute spines. The tongue is covered with rows of teeth curved backwards.
BRANCHIAL PLUME OF DORIS.The breathing organs consist of flat plumes, sometimes small and simple, sometimes large and branching, either united at the base into a flower-like expansion, or placed separately in a circle more or less perfect. In some species these plumes are capable of being withdrawn into a common cavity, the margin of which can be closed completely over them, but in others there is no cavity; and when danger threatens, the plumes are merely contracted, curled up, and bent down towards the centre of the circle.
TENTACLE OF DORIS.The upper or dorsal tentacles are generally short club-shaped organs, with the upper portion frequently bent into an angle. This portion is always set with narrow, oblique plates, usually ten or twelve in number, pointing behind and downward. They are always retractile, but for the most part not sheathed.
The species of this genus deposit their spawn in the spring; the eggs, as already described, being generally arranged in transverse rows, imbedded in a transparent gelatinous ribbon. Doris bilamellata, which I have kept in captivity, always deposited its ribbons on the side of the vessel just beneath the surface of the water. It adheres by one edge, and forms an imperfect spire or cup, the ribbon being bent upon itself, the upper edge or brim leaning a little outward, and being puckered. That of D. tuberculata is very large, the ribbon being three-fourths of an inch high, and wound round in many spiral coils. I have observed it, on the Devonshire shores, depending in a flaccid manner from the under surface of rocks, during the recess of the tide. The embryos in such coils of spawn are immensely numerous. Mr. Darwin thus describes the contents of a spawn-coil of a species of this genus, which he found at the Falkland Isles. "From two to five eggs [yolks], each 3⁄1000ths of an inch in diameter, were contained in a spherical little case [chorion]. These were arranged, two deep, in transverse rows, forming a ribbon. The ribbon adhered by its edge to the rock in an oval spire. One which I found measured nearly twenty inches in length, and half in breadth. By counting how many balls were contained in a tenth of an inch in the row, and how many rows in an equal length of the ribbon, on the most moderate computation there were six hundred thousand eggs. Yet this Doris was certainly not very common; although I was often searching under the stones, I saw only seven individuals."[2]
When we meet with accounts like this of the exceeding multitude of eggs produced by some species of animals, we are apt to wonder that the world is not filled with them, and to ask what becomes of these immense hosts. The fact is, they form the food of other creatures, a vast multitude—perhaps nine out of every ten—being devoured as soon as they are born. What Mr. Peach says of these very embryos, I have myself often observed in those that I have endeavoured to rear. They " have myriads of enemies in the small Infusoria, which may be noticed with a powerful microscope hovering round them, and ready to devour them the instant weakness or injury prevents their keeping in motion the cilia, which serve both for locomotion and defence. Let them cease to move, a regular attack is made, and the animal is soon devoured; and it is interesting to observe several of the scavengers sporting in the empty shell, as if in derision at the havoc they have made."[3]
The largest British species of this genus is D. tuberculata, often called the Sea Lemon. It attains three inches in length, is of a yellow colour, with the mantle covered with small round warts. Hence in size, form, hue, and surface, the animal bears no small resemblance to the half of a lemon cut through lengthways, especially when the branchial plumes and the dorsal tentacles are concealed by being withdrawn into their respective cavities. But the colour is not always plain yellow; it sometimes verges to grey, and is frequently marked in the most beautiful manner with large spots and clouds of crimson or purple. The branchial flower is composed of eight plumes, which are large, tripinnate, and of a bluish white hue.
SEA LEMON.
This species is said to be more common on the east coast of Scotland and England than on the west. I have, however, found it on the shores of Dorset and of north and south Devon; it lives among the rocks, in the zone bounded by the limits of high and low tide, as well as in deep water. In captivity, the habit of this, as of some other species of the genus, is to glide slowly round and round the vessel in which it is kept, just beneath the surface of the water, now and then lifting and puckering up the edges of the mantle, and allowing the air to bathe the sides of the body.
Family Tritoniadæ.
This is a small group considered in the number of its component species, though some of these are of unusually large size. Its distinctive characteristics are that the gills, which are either laminated, plumose, or papillose, are arranged down the sides of the back, and that the stomach is simple. By the former character it is distinguished from the Dorididæ, the feathery gill-leaves of which are retained in several of these, but never arranged in the form of a flower around the vent. By the latter it is severed from the Eolididæ, in which the stomach sends off branching tubes on each side.
There is no proper mantle distinguished as such from the general surface of the body; but there is often an elevated ridge running down each side, along which the gill-tufts are placed. In the Scyllæa, an oceanic genus found crawling among the stems and weeds of the floating gulf-weed, there are two or three erect, square lobes of flesh, projecting from each side of the back, and on the inner side of these the small tufted gills are scattered. In Glaucus, another oceanic animal of exquisite beauty, the gills take the form of fan-like pencils of filaments, diverging from the tips of long foot-stalks.
The head, in most of the species, is protected by a veil or expansion of membrane, sometimes cleft into tentacular processes, at others margined with a fringe. Tethys, another beautiful genus inhabiting the Mediterranean, has this veil enormously developed, and profusely fringed. The animal is said to use this organ to aid it in swimming, but this is hardly probable.
GLAUCUS.
So little has been recorded concerning the habits and economy of the marine, and especially the oceanic Mollusca, that I with pleasure quote the observations of Mr. George Bennett, on a species of Glaucus taken in the tropical part of the Atlantic ocean. The accompanying figure will convey an idea of the form of the animal, the colour of which is a vivid blue or purple on the upper surface, and silvery white beneath:—
"These little animals were very delicate and fragile in their structure, and although many—indeed I may say numbers—were caught, yet very few in comparison were found to be in a perfect condition, some being deficient in one, two, or more fins, and others being completely crushed. Not one of the specimens caught on this occasion, or during the voyage, had the silvery line or streak running down the back, from the head to the extremity of the tail, branching off also to the fins, and along the centre of each of the digitations. Several Porpitæ were also captured in the net at the same time with these animals, and served as food for them.
"It caused much regret to see the change death produced in the beauty of these interesting little animals, and all means of preserving them were found to be useless. When placed in spirits, the digits of the branchial fins speedily became retracted; the beautiful purple gradually faded, and at last disappeared, and the delicate pearly white of the under surface of the body and fins peeled off and disappeared. Thus did this beautiful Mollusk become decomposed in less than the space of an hour. Some Mollusks quickly lose their colour after death, but retain their form for a long time; but these speedily change after death, both in form and colour, and the beauty before so much admired perishes, never to be regained.
"When taken in the hand, the under surface of the animal soon becomes denuded of the beautiful pearly white it previously had, and at that time appears like a small transparent bladder, in which a number of air-bubbles are observed, together with the viscera. On the abdomen being laid open, a large quantity of air-bubbles escaped; and perhaps a query may arise how far they assist the animal in floating upon the surface of the water.
"I again placed several of the specimens in a glass of sea-water; they were full of life, sometimes moving about, not very briskly, however, and at other times remaining floating upon the surface of the water, merely gently moving the fins. As they floated upon the surface of the water in the glass, the sides of the head, back, tail, tins, &c. exhibited at the time a light silvery blue colour, which was admirably contrasted with the deeper blue of the upper surface, and falling into the elegant pearly or silvery white of the under surface of the animal, displaying an exceedingly rich and elegant appearance. Often, when at rest, the animal would, drop one or more of the fins; but on touching them, they would be immediately raised to the former position, and that organ was turned back as if to throw off the offending object, followed at the same time by a general movement of the whole body. On touching the animal upon the back, it seemed to display more sensitiveness in that than in any other part of the body, judging from the effects produced in comparison with similar experiments on other portions of the body. For instance, the centre of the back was touched lightly and rapidly with a feather, which caused the little creature to sink, as if under the pressure of the touch, throwing at the same time the head, tail, and all the fins upwards, followed by a general distortion of the whole body of the animal. As if the gentle touch had been productive of severe pain, I invariably found every part of the upper surface of the body very sensitive when touched, displaying a general movement of uneasiness throughout the whole of the body of the creature.
"These creatures have a peculiar manner of throwing the head towards the tail, and flouncing the tail towards the head, when they are desirous of removing any object of annoyance; it is at that time these animals seem to recover from their torpidity, and evince the greatest activity in their movements. When much annoyed, they throw the body about with great activity, coiling up the head, tail, fins, &c. in a somewhat rotundiform position, and if the tormenting object is not removed, dash out again in full activity of body, then return to the rotundiform position, and there remain for a short period, apparently exhausted by their efforts; but on the cessation of the irritating cause, the animal quietly resumed its original position, perhaps dropping one or two of its wearied fins, according as its own sensations of ease or comfort might dictate.
"When nothing irritated this tender Mollusk, it would remain tranquilly floating upon the surface of the water, with scarcely any movement but that which proceeded from the undulating motions of the digitated extremities of the fins, as well as an occasional slight twisting action of the same organs.
"I placed some small specimens of Porpita in the glass of water containing the Glauci, to observe if they would attack them. For some time one of the Glauci was close to a Porpita, and was even annoyed by the tentacles of the latter touching its back; yet the Glaucus bore this, although with the usual characters of impatience, yet without attempting to attack it. At last it seized the Porpita between its jaws, and by aid of a powerful lens, an excellent opportunity was afforded me of closely watching the devouring process, which was effected by an apparently sucking motion; and at this time all the digitated processes of the fins were floating about, as at other times, when the animal was at rest. But I did not observe, in one single instance, that they were of any use to the animals, either to aid in the capture, or to securely hold their prey when in the act of being devoured; for the animal seems to depend merely upon the mouth in capturing its prey, as, in this and other instances which I had opportunities of observing, they seized their prey instantly with the mouth, and held it by that power alone, whilst by a kind of sucking motion the prey was devoured. The digitations may, therefore, only be regarded as appendages to the fins, to aid the animal, perhaps, in the direction of its movements, as it was observed that they turned and twisted them about during the progressive motion—that is, when this tardy animal is pleased to progress,—as if in some way or other to direct the movements of the animal.
"The Glaucus, after eating the tentacles, and nearly the whole of the soft under surface of its prey, left the horny portion, and remained tranquilly reposing upon the surface of the water after its meal, the only motion visible in the animal being the playing of the digits of its fins. The mutilaterl remains of the Porpita sank to the bottom of the glass.
"Soon after, another Glaucus began a devouring attack upon another Porpita which had been placed in the glass, eating a little of it, and then ceasing after a short meal, occasionally renewing the attack at short intervals. On examining the Porpita which had been partially devoured by the ravenous Glaucus, I found the disc had been cleared of the tentacles and other soft parts, a small part of the fleshy portion only remaining upon the disc. Only one part of the horny disc exhibited any injury, and that appeared to be the place where the animal was first grasped by the Glaucus.
"When any of these animals came in contact with another in the glass, they did not display any annoyance, nor coil themselves up, nor did they evince any savage propensities one towards the other; and they would often float about having their digitated processes in contact one with the other, without exhibiting any signs of annoyance. Even when placed or pushed one against the other, they did not manifest any irritation, but remained undisturbed as in their usual moments of quiet repose.
"On the back of the animal being seen in a strong light, a black line could be discerned on each margin, and passing down the centre of each fin; and sometimes varied by having two black lines on the upper part of one fin, although the opposite fin may display but one.
"The margin between the falling of the purple colour of the back into the silvery white of the abdomen, often exhibited beautiful tints of a golden green; but these variations were probably produced by the effect of different rays of light. These animals soon perished; I could not preserve them for any length of time in the glass of sea- water, although the water was changed as often as it was thought necessary. The digitated processes of the fins were observed to shrink up on the death of the animal, and the process of decomposition rapidly took place, the whole body becoming a shapeless mass, having a bluish colour of deadly hue for a short period, and then becoming of a blackish or brownish black colour. I have seldom seen a gelatinous animal which appeared so firm whilst in the water, that proved so speedily to decompose when removed from it. Even the beautiful purple of the back, the silver or enamel of the abdomen, and the silvery blue of the sides, all speedily vanish, indeed instantly disappear, upon the death of the animal, as if it had been washed off; the expansive, delicate, and beautiful fins, and digitated processes, are no longer seen; they shrink up to nothing.
"Even on taking the animal alive out of the water, and placing it upon the hand, that instant almost, from its extreme delicacy, it was destroyed. The digitations of the fins fell off, the least movement destroyed the beauty of the animal, it speedily lost all the purple and silvery enamelled tints, and became a loathsome mass. Thus do we too often find animals, beautiful in external adornments, curious in their habits and organization, and calculated in every respect to supply us with inexhaustible sources of intellectual gratification, doomed speedily to perish—brief in the period allotted to them in the busy theatre of animated existence: but, doubtless, with the gift of existence, they have received from the bounteous hand of their Creator, the means of enjoying their fleeting lives.
"To place these little animals in the glass of water from the towing net, without injury to their delicate structure, required care; so that as soon as they were captured in the net, attached to the meshes, they were not handled, but carefully washed off, which was effected by dipping the meshes in the glass of water, when the animal soon detached itself without sustaining any injury, and floated in the water.
"Although these animals are so fragile, so easily destroyed on being taken out of their natural element, yet they fling themselves about in the water without sustaining any injury, without even the loss of any of the digitated processes of the fins; yet when there is much movement of the water, in carrying the glass from one place to another, they are evidently disturbed and restless, and the fins are dropped. If, therefore, a slight motion of the water disturbs them, what can become of these delicate Mollusks during tempestuous weather? Can they be similar to the delicate ephemera, doomed to live merely for the space of a day, and perish in myriads? From the immense number seen only from the ship,—and how many myriads more extended beyond our range of vision!—it conveyed to the mind some idea of the profusion of living beings inhabiting the wide expanse of ocean, and a feeling of astonishment at the inconceivable variety of forms and constructions to which animation has been imparted by creative power.
"The tail of this animal has been described as resembling that of a lizard. The comparison is good, not only with regard to form, but also (with, perhaps, a little more flexibility) to motion when in action. Sometimes the animal throws its tail up to the body, as if intended to brush off any annoying object, and at other times it has been observed to turn the head towards the side, as if for a similar purpose; it seems in the action of eating to resemble a caterpillar."[4]
Genus Tritonia.
The body is prismatic; that is to say, lengthened with ridges or angles running along it; it is often thick and firm. There are two tentacles, consisting of several branched filaments arranged in the form of a bundle or brush, and capable of being withdrawn into tubular sheaths. The head is protected by a veil sometimes cleft into filaments. The gills are small branching plumes, arranged in a single series along a ridge bordering each side of the back.
TRITONIA HOMBERGI.
The largest British species of the whole Order is a member of this genus, Tritonia Hombergi, which grows to six inches or more in length. It varies considerably in colour, being generally of a pink or purple hue, often varied with blue, but sometimes the entire animal is of an amber yellow.
This very fine species occurs all round our coast, but is considered somewhat rare. It seems to be a deep water species: I have procured it by trawling off Portland. It is remarkable for a power possessed only in a very limited degree by any of the Mollusca, that of producing an audible sound.
Professor Grant, to whom we are indebted for a record of the curious fact, says "that the sounds resemble very much the clink of a steel wire on the side of the jar, one stroke only being given at a time, and repeated at intervals of a minute or two. When placed in a large basin of water, the sound is much obscured, and is like that of a watch, one stroke being repeated as before at intervals. The sound is longest and oftenest repeated when the Tritoniæ are lively and moving about, and is not heard when they are cold and without any motion. In the dark, I have not observed any light emitted at the time of the stroke; no globule of air escapes to the surface of the water, nor is any ripple produced on the surface at the instant of the stroke. The sound, when in a glass vessel, is mellow and distinct."
The Professor has kept these Tritoniæ alive in his room for a month; and during the whole period of their confinement, they have continued to produce the sounds, with very little diminution of their original intensity. In a still apartment, they are audible at the distance of twelve feet. "The sounds obviously proceed from the mouth of the animal, and at the instant of the stroke we observe the lips suddenly separate, as if to allow the water to rush into a small vacuum formed within. As these animals are hermaphrodites, requiring mutual impregnation, the sounds may possibly be a means of communication between them; or if they be of an electric nature, they may be the means of defending from foreign enemies one of the most delicate, defenceless, and beautiful Gasteropods that inhabit the deep."[5]
MOUTH OF TRITONIA.The mouth in this animal forms, according to Cuvier's elaborate description, a cutting instrument of peculiar efficiency. "It consists of a large oval and fleshy mass enclosing the jaws and their muscles, as well as a tongue covered with spines, and its opening is guarded by two fleshy lips. The jaws form the basis of all this apparatus. Their substance is horny, their colour a yellowish brown, and their form very extraordinary, for an organ of this kind cannot be better described than by comparing it to the shears used in shearing sheep. They differ, however, in the following particulars: instead of playing upon a common spring, the two blades are formed to work upon a joint, and instead of being flat, they are slightly curved.
"The two blades are very sharp, and there is nothing that has life that they cannot cut, when the animal causes the cutting edges to glide over each other. For this purpose muscles of great strength are provided, the fibres of which are transverse; and their office is to approximate the two blades that are again separated by the natural elasticity of the articulation, whereby they are united at one extremity.
"The aliment, once cut by the jaws, is immediately seized by the papillæ of the tongue, which, being sharp and directed backwards, continually drag by a kind of peristaltic movement the alimentary materials into the œsophagus."[6]
Family Eolididæ.
The animals of this family have a peculiar elegance, delicacy, and beauty. They have the branchiæ arranged along the sides of the back. In one genus, which links this family with the preceding, these organs are branched, resembling little leafless trees; but in general they are warts of a long oval or spindle shape, pointed at the extremity. Their surface is covered with strong cilia, which constantly maintain a vigorous vibration, by the action of which currents of the surrounding water are continually poured along each of the papillæ, as these organs are named, affording the necessary oxygen for renewing the vital power of the blood. There is reason to believe, however, that the whole surface of the body, which is also covered with cilia, assists in respiration.
The papillae are permeated by a canal, which in many of the species is brilliantly coloured, contributing largely to the exquisite beauty of these little animals. This canal is connected with the stomach and the digestive function, supplying the place of the liver in other animals. In the principal genus of the family, each papilla is furnished with a curious organ; it is a little oval bag placed at the extremity of the papilla, and opening by a very minute aperture at its tip. Within the bag there are closely packed a vast multitude of clear oblong capsules, within each of which a narrow body is discerned running through its length. At the will of the animal, a number of these oblong capsules are ejected from the aperture, and at the same instant each capsule shoots forth with great force from one end a long thread or hair. These thread capsules are found in the Actiniæ and Coral animals, as also in Medusæ; and there can be little doubt that they are weapons of defence, and that the propulsion of the thread is the immediate cause of the stinging power possessed by many of these animals; probably, indeed, by all, though not appreciable by us.
PAPILLÆ OF EOLIS.
The structure of the mouth and of the tongue with its numerous teeth has been already described. A short gullet leads into a large membranous stomach, sending off on each side branches which subdivide and lead by smaller tubes into the canals, which I have already mentioned as running through the branchial papillæ.
Genus Eolis.
A vast number of species are comprised in this genus, and nearly fifty have already been discovered on our own shores. Their body is somewhat slug-like, sometimes broad, but more generally slender and much lengthened, without the least trace of a mantle. The skin is smooth, and not stiffened with spicula; the head is prolonged into two smooth tapering tentacles, which are apparently organs of touch, being waved about with great vivacity when the animal crawls; besides these there are two dorsal tentacles, situated as in Doris. Their surface is sometimes smooth, but sometimes set with membranous rings or oblique plates; the eyes are placed behind them. The branchiæ are lengthened, cylindrical, spindle-shaped, or flattened papillæ, arranged in transverse rows or clusters along the sides of the back, often very numerous. These organs usually project outwardly, leaving an open space down the middle of the back; they are commonly carried inclining backwards, and overlapping each other; but when the animal is alarmed or irritated, and particularly when it seizes its prey, they are stiffened, erected, and brought forward like the quills of a porcupine.
All the species are carnivorous, fierce, and voracious; some prey upon their weaker fellows, and even devour their own spawn. The ordinary food of most of the species appears to be the various kinds of zoophytes; some of the minuter sorts, creatures of great beauty, are almost invariably found nestling in the tiny shrubs of Sertularians, and there can be no doubt that they devour the polypes. Mr. Alder has seen E. coronata feed upon a Lucernaria, and I have found the largest of our native species, E. papillosa, eagerly gnawing the tentacles of Actiniæ more bulky than itself.
One of the most lovely of the species is the Crowned Eolis (E. coronata), which is scattered over most parts of our rocky shores; I have taken it in considerable number at Babbicombe, Devon, and likewise at Weymouth, clinging to the under surface of flat stones at extreme low water. When the stone is turned over, an inexperienced collector might readily overlook it, for it takes the appearance
THE CROWNED EOLIS.
of a shapeless knob of jelly about as large as a pea. On detaching it, however, and dropping it into a glass of clear sea-water, its beauty becomes apparent. It quickly unfolds itself into a slender, tapering animal, about an inch long, and of a clear pellucid appearance, tinged with pink. The papillæ are arranged in six or seven clusters on each side; they are slender, with the central canal of a rich crimson hue, the surface reflecting a brilliant metallic blue, and the tips opaque white. The tints of these organs are exceedingly beautiful; and as the animal moves them irregularly about, they shine with a radiance resembling that of rubies and sapphires.
These elegant branchiæ seem very easily dislodged; the specimens I have kept have usually lost one or more tufts, which, however, soon begin to sprout again. They are liable to be lost through the pugnacity of the animals themselves, as their predaceous habits frequently impel them to tear off each other's papillæ.
In captivity, this Eolis is very active, continually gliding with a uniform motion around the sides of the vessel, or climbing about the numerous branching sea-weeds that are growing in it. They frequently crawl close to the edge of the water, but never come actually out, though they occasionally float at the surface, by means of the expanded foot, back downwards.
Another species (E. punctata) has been heard to make that peculiar clicking sound, already mentioned as produced by Tritonia.
- ↑ Alder and Hancock's Monograph. (Doris.)
- ↑ Voyage of Adventure and Beagle, iii. 258.
- ↑ Annals of N.H, xv. 446.
- ↑ Proc. Zool. Soc. for 1836.
- ↑ Edin. Phil. Journ. xiv. 186.
- ↑ Jones' Anim. Kingd. 410.