CHAPTER IX.

The organization of the Echinodermata is the most perfect of all the zoophytes, serving as a transition between them and animals of more complicated frame. They have a digestive and vascular system, and a muscular system is almost always present; in short, they have internal or external respiratory organs, and a rudimentary nervous system has been detected in many of the species. The nutritive system is very simple, presenting in most of the family a single orifice in the centre of the lower surface of the body, destitute of teeth, performing the functions both of mouth and anus. De Blainville says that "the liver is apparent and rather considerable in the star-fishes, forming bunches occupying the whole circumference of the stomach, and extending to the cavities of the appendages where these exist." The mouth and gullet is admirably adapted for securing the testaceous mollusks and other substances on which they feed.

Reproduction in the Echinodermata appears to be monœcious. Ovaries are, as far as is known, the only organs of generation. They vary in number in different species. The sexes are usually separate: the young are produced by eggs, the embryo of which undergo important metamorphoses. Immediately after birth, the young asteriæ have a depressed and rounded body, with four club-shaped appendages or arms at their anterior extremity. When they are a little more developed, papillæ may be observed on the upper surface, in fine radiating rows: after twelve days the fine rays begin to increase, and after eight days more two rows of feet, or tentacula, are developed under each ray, which assist in the locomotion of the animal by alternate elongation and contraction, performing also the office of suckers. Like most other zoophytes, they have the power of reproducing parts of their bodies which may have been accidentally destroyed.

Asterias, or Star-fishes.

As to the animal which commonly and sometimes scientifically bears the name of Star-fish, in walking on the sea-shore at low tide, your eyes have often seen this strange creature half buried in the sand. It is so regular and geometrical in its form that it has more the appearance of being the production of man's hand than of a creation which breathes and moves. The Divine Geometrician who created it never realised a creature more regularly finished in shape, or more perfectly harmonious in symmetry.

The star-fish has five perfectly equal arms. They resemble a cross of honour, which has five branches. The star of the brave, the star of honour—these somewhat trivial words recall, nevertheless, the resemblance which exists between the two objects; doubtless, man has here taken Nature for his copy. It must, however, be remarked that, though five is the general number of lines in the star-fish, this number is not constant; it varies with different genera, species, and even with individuals. The connection of the arms with the disk presents equally remarkable differences. In the genus Culcita, the disk is so much developed that it constitutes, so to speak, the entire animal, whilst the arms form only a slight protuberance upon its circumference. In the genera Luidia, on the contrary, the disk is reduced to minimum, whilst the arms are of great length and very slender.

The colours of the star-fish vary greatly; they vary from a yellowish-grey, a yellow-orange, a garnet-red, to a dark violet, as their name indicates.

Star-fishes are exclusively and essentially beings of the sea; they are never seen in fresh water; they dwell amongst the submarine herbage, seeking for sandy coasts; they generally are found at moderate depths, but there are some species which are found at the great depth of a hundred and fifty fathoms.

Asterias are met with in almost every sea and under all latitudes, but they are most numerous and their forms are more richly varied in the seas of tropical regions. There are about a hundred and forty species described.

The body of the Asteria is supported by a calcareous envelope composed of juxta-posed pieces at once various and numerous. The number of these pieces is estimated at more than eleven thousand in the Red Sea Star-fish (Asterias rubens, Fig. 105), a species very common in Europe. The body of the Asterias rubens is likewise furnished with spines, granules, and tubercules, the shape, number, and disposition of which serve to characterise the genera and the species.

Another species, Asterias aurantiaca, will give an exact idea of the general type of animals of this order. This zoophyte, which is represented in Fig. 106, is common in the northern seas; it has five rather long arms, furnished with spines which are of an orange colour—hence its name. When we see one of these animals stranded upon the shore, it appears to be entirely destitute of all power of progression. But the star-fish is not always immovable; it is provided with an apparatus for locomotion, which appears to serve at the same time the purposes of respiration; for nature is not sparing in her gifts to the least organized beings; she bestows upon them feet, with respiratory organs, or lungs, which have the power of locomotion.

The muscular system, as already stated, is almost always present in the Echinodermata, but the organs of locomotion are very various, the principal being the membranous tubes usually termed feet, or ambulacra, which issue from the ambulacral apertures; but besides these, the rays themselves are movable, and in animals which are free to move from place to place these are used for the purpose. Thus in the common star-fish the rays may be bent towards the upper or lower surface of the disk, so as to facilitate its advance either in water over small spaces or up the vertical face of rocks. These ambulacra are very numerous, disposed in rows along the under surface of the rays; thus in A. aurantiaca there are two simple rows of feet attached to each ray, and the vesicular part is deeply cleft into two lobes; while in A. rubens (Fig. 105) there are two double rows on each ray, and each foot has an undivided vesicle.

Each of these ambulacra consists of two parts, an internal and generally vesicular portion placed within the body, and a tubular portion outside, projecting from the surface through an aperture in the skin or shell, the tube being closed at the extremity, and terminating in a sucker, usually in the form of a disk slightly depressed in the centre. The feet are thus muscular fleshy cylinders, hollow in the centre, and very extensible; by means of them the animal draws itself forward. The foot is extended by the contraction of its internal vesicle, which forces the fluid into the hollow tube, or, where the vesicle is wanting, by projecting the fluid into the tube by a communicating vessel. The tubular part is thus distended and elongated, and again retracts itself by means of its muscular fibres, by which action the fluid is forced back into the interior. In progression the animal extends a few of its feet, attaches its suckers to the rocks or stones, then, by shortening its feet, it draws its body forward. The progression of the Asterias is thus very slow, and so regular that only the closest observation enables the spectator to discover the movement which produces it. Like the movements of the hands of a watch, the eye cannot quite follow it. When an obstacle presents itself—if, for example, a stone comes in its way—it raises one of the rays in order to obtain a point of support, then a second ray, and, if necessary, a third,—and thus the animal creeps over the stone with as much ease as if it walked over the smooth sands. In the same way the animal creeps up perpendicular rocks, which is accomplished by means of these ambulacra and suckers. Frédol says: "If an Asteria is turned upon its back it will at first remain immovable, with its feet shut up. Soon, however, out come the feet, like so many little feelers; it moves them backward and forward, as if feeling for the ground; it soon inclines them towards the bottom of the vase, and fixes them one after the other. When it has a sufficient number attached the animal turns itself round. It is not impossible, whilst walking on the sea-shore, to have the pleasure of seeing one of these star-fishes walking upon the sand. A day rarely passes without one of them being thrown upon the strand by the tide, and then abandoned by the retreating waters. Generally they are left dead; this is not always the case, however; they are sometimes only benumbed. Place them in a vase full of sea-water, or simply in a pool on the shore, and you will sometimes see them recover from this death-like condition, and execute the curious movements of progression which we have described." The motions of an Asterias thus saved form a very curious spectacle.

The mouth of this animal is situated on the lower surface of the disk. At this point the constitutive pieces of the carapace leave a circular space, covered by a fibrous resistant membrane, pierced at the centre by a rounded opening. This opening is sometimes armed with hard papillæ, which play the part of teeth. The mouth almost directly abuts on the stomach, which is merely a globular sac, filling nearly all the central portion of the visceral cavity.

"Thus," says Mr. Milne Edwards, "in Asteracanthion glacialis the stomach is globulous, but imperfectly divided into two parts by a fold of its internal membrane; the first chamber, thus limited, appears to be more especially devoted to the transformation of the elementary matter into a liquid paste, which passes, in small portions, into the upper chamber. This is continued upward through a small intestine, and communicates laterally with five cylindrical prolongations, which each divide themselves again into two much elongated tubes, furnished with a double series of hollow branches, each terminating in a cul-de-sac." These organs advance into the interior of the rays or arms of the Asterias.

Imagine, then, an animal bearing digestive tubes in its arms—the same organ serving for digestion and progression. What lessons in economy does not the study of nature teach us! The products of digestion find an absorbent surface of great extent in the rays of the Asterias. They ought necessarily to pass rapidly from it into the circumjacent nourishing fluid.

The star-fishes are very voracious; they even attack mollusks which are covered with shells. M. Pouchett mentions having taken eighteen specimens of Venus intact, each being six lines in length, from the stomach of one large Asterias which he dissected upon the shores of the Mediterranean. It is now even said that the star-fishes eat many oysters.

Ancient naturalists were not ignorant that the star-fish was capable of eating oysters; but they believed that they waited for the moment when the bivalve would open its valves to introduce one of their rays into the opening. They imagined that having thus put one foot into the other's domicile, they soon put four, and finished by reaching and devouring the savoury inhabitant of the shell. Modern observations have modified the ideas of former naturalists upon this point. In order to obtain possession of and swallow an oyster, it appears that the star-fish begins its approaches by bringing its mouth to the closed edges of the oyster-shell; this done, with the assistance of a particular liquid which its mouth secretes, it injects a few drops of an acrid or venomous liquid into the interior of the oyster-shell, which forces it to open its valves. An entrance once obtained, it is not long before it is invaded and ravaged. Professor Rymer Jones gives another explanation of the transaction. According to this naturalist the oyster is seized between the rays of his ravisher, and held under his mouth by the aid of his suckers; the Asteria then inverts its stomach, according to the professor, and envelopes the entire oyster in its inmost recesses, while, doubtless, distilling a poisonous liquid. The victim is thus forced to open its shell, and becomes the prey of the enemy which envelopes it.

Whatever may be the modes of procedure employed by the star-fish, it is now clearly ascertained, however incredible the fact may at first appear, that it swallows oysters in the same manner as is practised at the oyster-shop.

This little being, formed of five arms and without any other apparent member, accomplishes a work which man is quite unable to execute—it opens an oyster without an oyster-knife.

If reasoning man had no other means of nourishment than oysters, and was without a knife to open them, it is very certain that with all his genius he would be puzzled how to get at the inaccessible and savoury bivalve so obstinately closed against him. The star-fish devours dead flesh of all kinds; their sole occupation is to feed themselves, and they keep up an incessant and active chase after all sorts of corrupt animal matter. The Asterias thus perform in the bosom of the sea the same part that certain birds and insects play on shore; they are its scavengers, and feed their bodies upon the carcases of animals which, if abandoned to the action of the elements, would become a cause of infection.

In the same manner that certain animals render the air healthy, the Asterias help, on a considerable scale, to keep the sea which shelters them in a pure and healthy state. Zoologists are not agreed upon the manner in which respiration operates on the star-fishes. Nevertheless they think that the principal part in this phenomenon devolves upon the subcutaneous branchiæ which in each ray constitute two double series of bladders. The function of circulation is equally unknown. The vascular apparatus is sufficiently developed in this zoophyte, and appears to have for its centre an elongated canal with muscular walls, which may with justice be honoured with the name of heart. A little ring surrounding the œsophagus, and from which issue certain delicate white chords, which are prolonged into the furrows of the arms, presents us with all that can be designated a nervous system in the star-fishes. Among organs of sense we may mention, as the apparatus of touch, the tentacular ambulacra, as well as those which are disseminated upon the dorsal surface of the disk. The eyes are considered to be certain bright red points which are situated at the extremity of the arms and on the under surface—a most singular position for the organs of sight. The eyes must, besides, be very imperfect, for they possess no crystalline lens. Ehrenberg insists upon the existence of eyes in some species, attributing the function to those red spots, however; while Rymer Jones attributes the indications in which this originates to an extremely delicate sense of touch in the star-fishes. Professor Edward Forbes, while he admits the existence of ganglions in the nervous system to be extremely doubtful, seems, by the frequent use of the terms eye and eyelids, to admit that the specks in question are visual organs; the weight of authority inclines therefore to Ehrenberg's view, that if not eyes in the strict sense of the term, they serve the purposes of vision, modified and adapted to the wants of the animal.

The star-fishes have distinct sexes, with individual differences; their eggs, which are round and reddish, undergo curious phases of development. They produce little worm-like creatures, covered with vibratile hairs, like the infusoria, which swim about with great vivacity; these little creatures are subject to considerable changes. In the year 1835 M. Sars described, under the name of Bipinnaria asterigera, an enigmatical animal resembling a polyp from the arms at one extremity of the body, while the other terminated in a tail, furnished with two fins; but it was chiefly remarkable as having an Asterias attached to the extremity which carried the arm. He expressed an opinion, which was soon placed beyond any doubt, that this bipinnaria was an Asterias in its course of development. The egg becomes a sort of infusoria, the infusoria becomes a bipinnaria, and this produces the Asterias. In short, the Bipinnaria does not become an Asterias by any metamorphoses analogous to that so well known amongst insects—the butterfly, for example—but becomes, so to speak, the foster-mother or nurse to the Bipinnaria. The larva is large, and it is at the cost of a very small internal rudiment of this larva that the Asterias is developed: the Asterias robs the larva of its stomach and intestines, and turns it into a visceral apparatus for its own use. But the Asterias makes itself a mouth of any of the pieces most remote from the primitive mouth of the larva. Thus the Bipinnaria divides itself; it gives its stomach and intestines, and keeps its œsophagus and mouth, and it can live several days after the Asterias is detached from it.

Can any one imagine the existence of a being with only a mouth and œsophagus, which has neither stomach nor intestines, because another animal has possessed itself of them for its own use? The study of the lower animals abounds in surprises of this kind. It is a chain of unforeseen facts; of natural impossibilities; of realized points necessarily reversing all notions obtained in the study of beings which have a higher place in the animal scale. The history of the star-fishes would be incomplete were we to omit mentioning the most remarkable traits of their organisation with which naturalists are acquainted. The animals exhibit in the highest degree the vital phenomena of dismemberment and restoration, that is to say, of the faculty of reconstructing organs which they have lost. These arms, the structure of which is so complicated, and which protect such important organs, may be destroyed by accident. The animal troubles itself little at this mutilation: if he loses an arm it disquiets him but little; another is immediately procured. We often see in our collections of Asterias specimens wanting in symmetry because they have been taken before the new members which are in process of development have attained their definite length. Professor Rymer Jones mentions an instance of redintegration very complete and most curious. This naturalist had an isolated ray of Asterias which he had picked up; at the end of five days he observed that four little rays and a mouth had been produced; at the end of a month the old ray was completely destroyed, and this apparently useless fragment had been replaced by a new being, quite perfect, with four little symmetrical branches. This faculty of reproducing organs, which we have noted in describing the fresh water polyps, the sea anemone, &c., exists also in many other zoophytes, but in none more strikingly than in the Asterias. But a still more startling fact remains to be mentioned: one more strange and more mysterious, for it does not belong to the physical or organic order, but appears to belong to the moral world. The star-fishes commit suicide! Certain of these animals appear to escape from dangers which menace them by self-destruction. This power of putting an end to existence we only find on the highest and lowest steps of the animal scale. Man and the star-fishes have a common moral platform, and it is that of self-destruction! This power of dismemberment, however, seems to be confined to the Ophiocoma and Luidia—at least, it is only carried out to its full extent in these generæ.

Mysteries of Nature, who can sound your depths? Secrets of the moral world, what being but God has the privilege of comprehending you? A large species of Star-fish (Luidia fragillissima), which inhabits the English seas, has this instinct of suicide to a great extent. The following account by Professor Edward Forbes of an attempt to capture a Luidia gives a good illustration of its powers. "The first time that I took one of these creatures," the professor says, "I succeeded in placing it entire in my boat. Not having seen one before, and being ignorant of its suicidal powers, I spread it out on a rowing bench, the better to admire its form and colours. On attempting to remove it for preservation, to my horror and disappointment I found only an assemblage of detached members. My conservative endeavours were all neutralised by its destructive exertions; and the animal is now badly represented in my cabinet by a diskless arm and an armless disk. Next time I went to dredge at the same spot I determined not to be cheated out of my specimen a second time. I carried with me a bucket of fresh water, for which the star-fishes evince a great antipathy. As I hoped, a Luidia soon came up in the dredge—a most gorgeous specimen. As the animal does not generally break up until it is raised to the surface of the sea, I carefully and anxiously plunged my bucket to a level with the dredge's mouth, and softly introduced the Luidia into the fresh water. Whether the cold was too much for it, or the sight of the bucket was too terrific, I do not know; but in a moment it began to dissolve its corporation, and I saw its limbs escaping through every mesh of the dredge. In my despair I seized the largest piece, and brought up the extremity of an arm with its terminal eye, the spinous eyelid of which opened and closed with something exceedingly like a wink of derision."

The mind remains confounded before such spectacles, and we can only say, with Mallebranche, "It is well to comprehend clearly that there are some things which are absolutely incomprehensible."

This is doubtless the reason that in collections of natural history we rarely find star-fishes, and especially the Luidia, entire; the moment the animal is seized by fisherman or amateur, in its terror or despair it breaks itself up into small fragments. To preserve them whole they must be killed suddenly, before they have time to be aware of their danger. For this purpose, the moment they are drawn from the sea they must be plunged into a vase of cold fresh water; this saltless liquid is instant death to these creatures, which in this condition perish suddenly before they have time to mutilate themselves. The star-fish is a curious ornament in our natural history collections, but in this state they represent very imperfectly the elegance and particular grace of this curious type. To understand the star-fishes, they must be seen in an aquarium, where we can admire the form, figure, movements, and manners of these marvellous beings.

The Asterias are the planets of the sea. It may be said that heaven, reflected during the night on the silvery surface of the ocean, let fall some of those stars into its depths which decorate the resplendent vault.

Crinoïdea.

We quoted the maxim of Linnæus in the earlier pages of this volume, that Nature makes no leaps. Nature proceeds by means of insensible transitions, rising by degrees from one organic form to another. Most of the animals hitherto described are immovably fixed to some solid object; at least, such is their condition in the adult state. We are about to describe zoophytes free of all fetters; animals "which walk in their strength and liberty."

Between zoophytes fixed to the soil, like the corals, gorgons, and aggregate zoophytes, such as sea-urchins and holothurias, Nature has placed an intermediate race, namely, the Crinoïdea, a class of zoophytes which are attached to a rock by a sort of root armed with claws, having a long flexible stem, which enables them to execute movements in the circle limited only by the length of this stem, just as the ox or goat in our paddocks is confined by its tether to the space circumscribed by the length of its rope.

Let the reader picture to himself a star-fish borne upon the summit of a flexible stem firmly rooted in the soil, and he has a general idea of the zoophytes which compose the order of the Crinoïdea. Naturalists of the seventeenth century bestowed the name of stone lilies on these curious products. This rather poetical name proves that the conformation of these creatures had at an early period attracted observation, presenting the naturalist with the most curious of his lessons. The encrinites raise, as from the dead, a whole world buried in the abyss of the past. At the present time only two genera of these zoophytes exist, whilst in the early ages of the world the ocean must have swarmed with them. Encrinites abounded in the seas during the transition and secondary epoch. It was one of the most numerous of the animal tribes which inhabited the salt waters of the ancient world. In traversing some parts of France, we tread under our feet myriads of these beings, whose calcareous remains form vast beds of rock. The encrinites gradually disappeared from the ancient seas; their species were diminished as the globe became older or modified in its conditions, so that at the present time only a few types remain in our seas—such as the Comatula of the Mediterranean; Pentacrinus, the Medusa's-head of the Antilles; and the European Pentacrinus—all of them very rare, and probably destined soon to disappear, carrying with them the last reminiscence of the zoological races of the ancient world: and here lies the real interest which the Crinoïdea presents to the thinking man. The encrinites most common in the fossil state are Pentacrinus fasciculosus, belonging to the lias; Apiocrinus rotundus, which is found in the oolite or jurassic rocks; and Encrinus liliformis, which appertains to the Triassic period. These three fixed zoophytes seem to have existed in great numbers during an early age of the world—namely, the Silurian period. They attained their maximum of development during the Devonian age, after which they begin to decrease. According to M. D'Orbigny, there are thirty-nine genera found in the palæozoic rocks, two in the triassic, seven in the jurassic, five in the cretaceous, and only one in the tertiary strata. Of all these genera only one, namely, Pentacrinus, is found in the modern epoch to represent the varied forms of these the first inhabitants of the seas.

The free Crinoïdæ, that is, those not rooted to the soil by a stem, of which the Comatula may be considered the type, only appeared at a later period. They are absent in the palæozoic and triassic rocks, but appear to have attained their maximum of development in the jurassic period.

The numerous fossilized remains of these curious creations, which abound in different rocks, attracted the attention of learned men at an early period. The encrinites were among the earliest objects of scientific description. As early as the sixteenth century, the celebrated mineralogist, George Agricola, mentions them under the names of Entrochites, Trochites, and Astroïtes. At the same time, and since that epoch, the Crinoïdæ, which we know by the name of stone-lilies, and which characterises the Muschelkalk rocks, have been known under the name of Encrinus, from εν, stone, and κρίνον, a lily.

During the eighteenth century the works upon the Crinoïdæ were very numerous, though not very correct. They sometimes reported these organic remains to be vegetable; sometimes they were beings allied to the star-fishes; at others they were the vertebral column of fishes. Towards the year 1761, however, Guettard, one of the most learned naturalists of his time, understood the real nature of these productions. He had occasion to examine a recent Encrinus sent from Martinique under the name of Sea-Palm, which was in reality Pentacrinus caput Medusæ. The comparison of the living individual with the fossil fragment described by his predecessors, and of which he had specimens in his collections, enabled him to ascertain the real origin of the fossil Encrinoidæ. The beautiful fragment which still exists in the Museum of Natural History at Paris was long considered unique, but it is now known that ten others exist in different museums. Since that date the Crinoïdæ have been examined and described by observers such as Miller, Forbes, D'Orbigny, and Pictet, and very elaborately by Major Austin.

"The species of fixed Crinoïdæ actually living are Pentacrinus caput Medusæ (Fig. 107), and Pentacrinus Europæus (Fig. 108). These curious zoophytes resemble a flower borne upon a stem, which terminates in an organ called the calyx, but which is, properly speaking, the head of the animal. Arms, more or less branching, spring from this calyx, their ramifications, so formed, consisting of many pieces articulated to each other. The calyx is supported by a stem, varying in height, formed of pieces secreted by the living tissues which surround them. The articulations of this stem are usually very numerous, cylindrical, and present a series of rays striated upon their articulated faces. In Pentacrinus they are prismatic and pentagonal; that is, they present five projecting angles, and on their articulated face a star with five branches, or, better still, a rose with five petals. At the base of the stem of this animal-plant, in many of the Crinoïdæ, we find a sort of spreading root, which is implanted in the rocks, and is capable of growing by itself, of nourishing the stem, and of producing new ones.

The root and stem of the fixed encrinites seem to indicate that the animal can only live with the head erect. Their normal condition is thus quite different from that of any other of the Echinoderms, almost all of which keep their mouths invariably directed downwards.

The Medusæ heads are chiefly found on rocky beds, or in the midst of banks of corals, at great depths. There, firmly fixed by their roots, their long stems raise themselves vertically; then, with expanded calyx and long-spreading arms, they wait for the prey which passes within their reach in order to seize it.

The Pentacrinus caput Medusæ have, as we have said, been fished up from great depths in the Antilles. Its very small calyx is borne upon a stem of from eighteen to twenty inches in height, terminating in long movable arms, the internal surface of which bears its tentacles in a groove. In the middle of the arms is a mouth, and at the side the orifice for the expulsion of the digested residuum.

In the Medusæ head and European Pentacrine (P. Europæus, Fig. 108), the presence of a digestive apparatus has been distinctly traced. It is a sort of irregular sac, with a central mouth on the upper surface, and another orifice situated at a little distance from the mouth, and evidently intended as an outlet for the products of digestion. The arms of these creatures, which are spreading or folded up according to their wants, are provided with fleshy tentacula, which, serving at once as organs of absorption and as vibratile cilia, are at the same time organs of respiration. Such are these curious beings: they occupy a sort of middle or transition state between animals permanently fixed to some spot and those capable of motion, representing in our own times the last remains of extinct generations. Every type of the Crinoïdæ furnished with arms presents incontestable evidence of their mode of reproduction or redintegration—that is, of the power of restoring those parts of the body broken or destroyed by accident; but as we have already drawn the attention of the reader to this strange faculty of renewing organs which many of the zoophytes possess, we will not here enlarge further upon the subject.

The Crinoïdæ are not all like the two species which have been described. There is an entire family of animals belonging to this class, namely, the Comatula, which are fixed in their early days, but separate themselves from the rooted stem in their adult age, and, throwing off the bonds imposed on their youth, live side by side with the asterias, with whose company they seem much pleased. The encrinites and the star-fishes thus live in company, and that at prodigious depths, and under a body of water which no light can reach. Imagine the existence of animals which pass their lives in such eternal funereal darkness. The family of Comatula are found in the seas of both hemispheres. Their bodies are flat—a large calcareous plate formed like a cuirass upon their backs—presenting, besides, cirri composed of numerous curling articulations, the last of which terminates in a hook. The ventral surface presents two orifices: the one in the centre corresponding to a mouth, the other evidently intended for the discharge of the products of digestion. This animal is provided with five arms, which diverge directly from the centre plate or cuirass. The branches of these arms have ambulacral grooves, comprehending a double row of fleshy tentacles, in the centre of which is the ambulacral groove, properly so called, clothed with vibratile cilia over their whole surface. These cilia or hairs guide the current which drives the various substances on which it feeds, such as the organic corpuscles of sea-weeds, and microscopic animalcules floating in the sea, towards its mouth. They are also powerful aids to respiration.

The movements of these curious creatures are very slow, their only object being to catch the bodies of animals and marine plants, or, by extending or contracting their arms, to feel their way through the water to some new locality. Sometimes, also, in order to change their feeding-ground, the Comatula abandon the submarine forests, herbage, and sea-wracks, and float through the water, moving their arms with considerable rapidity in search of a new station.

The Mediterranean Comatula (Fig. 109) is largely diffused on the European shores of the Mediterranean. Its spreading arms extend to three or four inches; its colour purple, shaded, and spotted with white upon the ventral surface.

Were a traveller to tell us that he had seen animals drop their eggs upon forests of stone; that these eggs, after executing their progressive evolutions, finally become individuals in all respects like their parents, which attach themselves to the soil by a root like any flower of the fields, or to the mother-stem like the branch of a tree, until in due course they attained the adult state, when the flexible band which holds them fixed either to the soil or parent-stem breaks, and the animal, now free, launches itself into the liquid medium, and goes to live a proper and independent existence;—in listening to a recital so opposed in appearance to the ordinary laws of Nature, we should be inclined to tax the narrator of such incredible facts with error or folly. Nevertheless all these facts are now perfectly established. The being which presents these marvels has nothing of the fabulous about it. It is the Comatula Mediterranea; it lives at the bottom of the sea, the surface of which is incessantly tracked by our vessels.

Ophiuradæ.

The Ophiuras are thus named from two Greek words (ὅϕις, a serpent, and οὑρὰ, a tail), from their fancied resemblance to the tail of a serpent. These zoophytes are met with in almost every sea, but chiefly in those of temperate regions; they are very common on every shore, and have been remarked by fishermen from the earliest times on account of their singular form, the disposition of their arms, which resemble the tail of a lizard, and by the singularity of their movements. The general characteristics of this remarkable group of Echinodermata, as described by Dujardin and Hupé, are as follows. They are radiary marine animals creeping at the bottom of the sea, or upon marine plants. In form they present a sort of coriaceous disk, which is either bare or covered with scales, which contains all the viscera, and five very flexible simple or branching arms, each sustained by a series of vertebral internal pieces, naked or covered with granules, scales, or bristles. Certain fleshy tentacula thrown out laterally are organs of respiration. The mouth is situated in the middle of the lower surface of the disk, and opens directly into a stomach in the shape of a sac; it is circumscribed by five re-entering angles corresponding with the intervals of the arms, having a series of calcareous pieces, which perform the function of jaw-bones. This mouth is prolonged by five longitudinal clefts, garnished with papillæ or calcareous pieces, which correspond to one of the arms. A series of calcareous pieces in the shape of vertebræ spring from the extremity of each of these clefts, which occupy all the interior of the arms, having a furrow in the middle of the ventral surface for the reception of a nursing vessel; and laterally between their expansions are certain cavities, from whence issue certain fleshy retractile tentacula; the visceral cavity opens by one or two clefts on the ventral surface of each side of the base of the arms.

The Ophiuradæ move themselves by briskly contracting their arms so as to produce a succession of undulations analogous to those by which a serpent creeps along. Some of these zoophytes are rather active; but others attach themselves by their arms to the branches of certain other polyps, like the Gorgons, and remain immovable for a considerable time, waiting their prey somewhat like a spider in the midst of his web.

The family of Ophiuradæ is divided into two great sections: that of the Ophiura, which comprehends several genera, amongst others that which gives its name to the family, and that of the Euryalina or Asterophytes.

The family of Ophiuradæ constitute a group distinguished by their five simple, articulated, very mobile, and non-ramified arms, which are attached to a small disk or shield plate, with flexible thread-like cirri between the rays. Ophiura natta is very common, and has been known from very early times in European seas. It is of a greenish colour, with transverse bands, which become more obscure upon the arms as the distance from the disk increases. This disk is from six to seven-eighths of an inch in size, the upper part covered with unequal plates, in shape like tiles; the arms are four times the length of the diameter of the disk, very slender and tapering. The zoophyte to which Lamarck gave the name of Ophiura fragile has now its place among the Ophisthrix, the specific name, indicating a particularity of structure in all these small creatures derived from their fragile formation. In short, these beings have so little consistency that they crumble, as it were, under the touch, and become reduced to pulp under the slightest pressure. In Fig. 110 we give the representation of an Ophiura of the natural size, which Lutken has since called Ophiocoma Russei. This Echinoderm, which lives in the seas of the Antilles, is furnished with five very flexible rays, which are armed with from three to four rows of spines, those on the upper part of the body being very hard ones; the body and arms of this creature are of reddish brown, streaked with a great number of little white lines.

The principal type of the Euryalina is the curious and complex Asterophyton verrucosum of Lamarck. They include animals remarkable for the extremely complicated development of their arms—the very multiplied ramifications of these, towards the extremities, being divided into many thousand very slender appendages, the principal use of which is doubtless locomotion, but at the same time they constitute a series of living thread-like fillets which seem intended to seize and close upon the animals which serve as prey to this little flesh-eater. The Asterophyton verrucosum, which is represented in Fig. 111, is yellowish; its disk about four inches, its arms sixteen to eighteen. It inhabits the Indian Ocean. Another species, Euryala arborescens, is met with on the coasts of Sicily and other parts of the Mediterranean. Nothing can be more elegant than these animated disks, which resemble nothing so much as a delicate piece of lace—a piece of living lace moving in delicate festoons in the bosom of the ocean.

Echinidæ.

The singular shape of the Echinidæ, or Sea-urchins, and the spiny prolongations with which their bodies are covered, has in all ages attracted the attention of naturalists. Aristotle applied to them the name ἐχῖνοϛ, which signifies urchin. When, however, one sees the body of one of these animals thrown on the sea shore, it is difficult, at first, to find a reason for this designation. The body of the sea-urchin is furnished with a species of spine. It is a sort of shell, nearly spherical, empty in the interior, its surface presenting reliefs admirable for their regularity—an egg-shell sculptured by Divine hands. In order to see the urchin with its spines, it is necessary to seize it in the water at the bottom of the sea, where it rolls and moves its little prickly mass; it is then only that the real urchin, the prickly sea-urchin, is to be seen, bristling with prickles, and strongly resembling, to compare the physical with the mental, those amiable mortals whose character is so well depicted in the saying, "Whom they rub they prick."

In his book on "The Sea," Michelet puts the following conversation into the mouth of a sea-urchin:

"I am born without ambition," says the modest Echinoderm. "I ask for none of the brilliant gifts possessed by those gentlemen the molluscs. I would neither make mother-of-pearl nor pearls; I have no wish for brilliant colours, a luxury which would point me out; still less do I desire the grace of your giddy Medusas, the waving charm of whose flaming locks attracts observation and exposes one to shipwreck. Oh mother! I wish for one thing only: to be—to be without these exterior and compromising appendages; to be thick-set, strong, and round, for that is the shape in which I should be the least exposed; in short, to be a centralized being. I have very little instinct for travel. To roll sometimes from the surface to the bottom of the sea is enough of travel for me. Glued firmly to my rock, I could there solve the problem, the solution of which your future favourite, man, seeks for in vain—that of safety. To strictly exclude enemies and admit all friends, especially water, air, and light, would, I know, cost me some labour and constant effort. Covered with movable spines, enemies will avoid me. Now, bristling like a bear, they call me an urchin."