Chapter IX: Echinodermata (2)

The shape of the _Clypeaster rosaceus_ is oval, straighter in front, and thick and rounded at the edges. It is more common and more largely distributed than any other living species, and it is supplied with four or six ambulacra, or feet.

I never could understand why the dental framework of the sea-urchin has been called Aristotle's Lantern, for this formidable apparatus resembles the front view of a battery of cannon more than a lantern. It consists of a series of pieces designated by the names of compass, scythe, pyramid, and plumula, which it would serve no useful purpose to describe.

We have said that the mouth of the urchin is monstrous in proportion to its size, and the teeth of proportionate dimensions. As these project from a very formidable mouth, one can easily be assured of the sharpness of their extremities by intruding his fingers on them. In fact, it is necessary that these organs should be singularly powerful, because, as we shall see farther on, the sea-urchin makes incisions in the solid rock with them, and hollows out shelter for himself. The strong and sharp teeth grow at the base in proportion as they are used at the points, as is the case with some of the rodent mammalia. By this means they are always sharp and in good condition. Five groups of powerful muscles are used to work these terrible grinders.

To this formidable mouth is attached an œsophagus or gullet, and an intestine which extends along the interior walls of the carapace, describing the circumference of its principal contour.

The regimen of the Echinidæ is still imperfectly known; nevertheless, from the presence of shells, fragments of corals, crustaceans, and even other Echinodermata in their intestinal tube, it is to be inferred that a certain number of them at least are carnassiers, or flesh-eaters, while others are supposed on the same evidence to be vegetarians. The organs of respiration of the Echinidæ appear to be certain flattened vesicles in the form of very delicate laminæ, which adhere to the internal surface of the walls of the body, and float freely in the liquid with which the visceral cavity is filled. These organs, known as the internal _branchiæ_, are in communication with the central canal and ambulacral tubes. The heart is spindle-shaped, tapering above, swelling below. There are two distinct vascular systems, one intestinal, the other cutaneous.

Their nervous system consists of a ring, which surrounds the gullet, and is placed at a short distance from the mouth. In this ring the nervous trunks have their origin. In relation to the senses, that of touch is highly developed. Certain branching tentacula, which surround the mouth, fashioned like nippers, and the ambulacral tentacles, are its principal organs. They appear to be altogether destitute of organs of sight. It has sometimes been argued that four or five red points at the summit of the dorsal face are eyes; but this opinion has not been maintained, nor has any crystalline lens been found in these spots to justify it. Captain de Condé states that he examined a sea-urchin with long spears in a pool of water, which he tried to catch, when he saw it direct its flight towards his hand, all its defences being erect. Surprised at this manœuvre, he tried to seize it from another quarter; its spines were instantly directed to the other side. "I have thought from that time that the urchin saw me, and prepared to resist my attack. In order, however, to satisfy myself whether or not the movement in the water caused by my approach might have produced the effect described, I repeated the experiment with greater caution. But the creature always directed its spines in the direction of the object which threatened it, whether it was in the water or out of it." He satisfied himself that these animals certainly could see, and that their spines served them as a means of defence.

These wonderful spines, this calcareous envelope, this armour so marvellously studded, with which nature has so bountifully provided the Echinidæ, appear to have been insufficient, inasmuch as these very spines, in order to secure the safety of the animal, are gifted with the power of hollowing a dwelling for themselves out of solid rocks of the hardest material, such as granite and sandstone. They fix themselves to its surface by means of their tentacles; they make an incision by means of their strong teeth, removing the débris with their spines as fast as it is produced. When the hole is large enough, they entrench themselves in it, with their spines and their threatening pikes levelled to protect them from all external assaults. To M. Caillaud, the conservator of the museum of Nantes, we are indebted for an excellent account of the manner in which this buccal apparatus is made to operate. "The Lantern of Aristotle," says this author, "forms the mandibullary apparatus; the teeth are five in number, and they may as well receive the denomination of a series of saws and picks as of teeth, for they are surprisingly adapted to the excavation of holes in the hardest rock. These five picks are about the eighth of an inch long, and they serve the sea-urchin at once as masticators and excavating implements. In opening the jaws, these five teeth strike the stone forcibly rather than scrape it." This property of hollowing their dwelling out of the solid rock appears, however, to belong to only a small number of the Echinidæ; most of them are content to hide themselves under the stones, while the species having the spines slender and the shell very thin bury themselves in the sand, with which they cover themselves entirely, leaving only a small hole to breathe through. The _Spatangus_, which is furnished with short thick spines on the under part of its body, which spread out at the extremity like the channel of a spoon, proceeds with its mining operations as follows, according to Mr. Jonathan Franklin. "Figure to yourself, reader, the animal on the sea-shore. He commences his operations by turning the lower spines in such a manner as to form a hollow on the sand bank, in which he sinks by his own weight; but as he sinks, a great number of the spines are brought into action, throwing up the sand with increased activity, while the sand thrown up, returning again, soon covers the body of the worker, and he has soon buried himself beneath the surface. In this situation the long hair-like spines situated upon the back begin to play their part; they prevent the sand from entirely covering the animal by forming a little round hole, through which water is introduced to the mouth and respiratory organs." The hiding-place of the sea-urchin is, however, easily detected in the sand by the hole thus arranged for the respiration of the animal, and the fishermen think they can predict storms according to the depth of the hole.

The Echinidæ are reproduced by eggs, which are red and nearly microscopic. As it issues from the egg the larva has the appearance of a very minute fish. It is not at once converted into the perfect animal, but undergoes a certain metamorphosis analogous to that of the caterpillar into the butterfly. But, as we have already stated in treating of the Asteriæ, it produces, at a certain stage, by some sort of internal process of generation, a sea-urchin, which, being at first only an organ of the larva, begins to live an independent life when the nursing larva has destroyed itself. The manner in which the urchin unfolds itself at the expense of the larva is quite analogous to that which the asterias present: it is another case of alternate generation, of which our space does not permit us to give even a general outline.

Sea-urchins are found in every sea; they dwell in sandy bottoms, and sometimes upon rocky ground. They are caught with wooden pincers when in shallow water; when found at the water's edge, they may be taken by a gloved hand.

The urchin, like the crab, which it also resembles in taste, becomes red when boiled; only certain species are comestible, however. In Corsica and Algeria the Melon-shaped Urchin (_Echinus melo_) is much esteemed. In Naples and in the French ports of the Channel the _Echinus lividus_ is eaten. In Provence the Common Sea-urchin (_Echinus esculentus_ and _Echinus granulosus_) are the favourites.

Sea-urchins are eaten raw like oysters. They are cut in four parts, and the flesh taken out with a spoon; they are sometimes, but more rarely, dressed by boiling, and eaten from the shell like an egg, using long sippets of bread: hence the name of sea-eggs, which they bear in many countries.

Sea-eggs were a choice dish upon the tables of the Greeks and Romans; they were then served up with vinegar or hydromel, with the addition of mint or parsley. When Lentulus feasted the priest of Mars--the Flamen Martialis--this formed the first dish at supper. Sea-eggs also appeared at the marriage feast of the goddess Hebe. "Afterwards," says the poet, "came crabs and sea-urchins, which do not swim in the sea, but content themselves by travelling on the sandy shore." For my own part, I have only once partaken of sea-urchin, and it appeared to me to be food fit for the gods; but perhaps the circumstances sufficiently explain this dash of culinary enthusiasm. The Reserve Restaurant at Marseilles has not always been the vast stone edifice we now behold, backed majestically by the mountain, and fronting the sea on the promenade of the Corniche du Prado. In 1845 it rose quite at the entrance of the port, a small glass cage, suspended as it were by a magic thread between the heavens and the sea. From this aërial dwelling, overhanging with unheard-of audacity the waters which surrounded it on all sides, we gazed on the most wonderful prospect in the world, and reposed ourselves while enjoying this intoxicating scene, during which the ships were continually entering the port, passing under our very feet. It was in this enchanted palace that sea-urchins were served up, supported by the traditional bouillabaise.

As I have said, it appeared to me delicious. Was it the Provençal dish, the savoury bouillabaise, which contributed to my appreciation of the humble sea-urchin of the Mediterranean? Was not the marvellous view which I enjoyed from the heights of my empyreum of glass the indirect cause of it? This is a tender and charming problem which I love to leave floating in the clouds, half evanescent, of my youthful recollections.

HOLOTHURIA.

The ignorant, like you and I, call the Holothuria the Cornechou, or Sea-cucumber, and perhaps, for two reasons, they are not far wrong. The term sea-cucumber expresses with wonderful exactness the shape of the animal, and its habitation, the sea; and, again, it would puzzle the most learned to explain the word _Holothuria_. The body of this strange creature presents the form of an elongated and worm-like cylinder; its dimensions are so variable that, while some species are only an inch or two in length, others attain thirty and even forty. In general, the skin of the Holothuria is thick and leathery; it includes muscles, and is armed occasionally with small projecting hooks or fangs, which enable the creature to hang for a few seconds on to foreign bodies. From this coriaceous envelope issue tentacular feet analogous to those described in the sea-urchin and sea-star.

When we open a Holothuria we find nearly the whole internal cavity occupied with little white tubes. We know that the fabulous cucumber spoken of in the "Arabian Nights" was stuffed with pearls by the talking-bird. With our poor animal this, alas! is not so. These are no pearls, but simple prosaical tubes containing the ova. The mouth opens at the extremity of the body; it forms a sort of funnel, and is surrounded, as by a glory, with an elegant circle of tentacula. In the living animal, when it feels itself in security, these tentacles expand themselves like the corolla of a flower. When the fisherman seizes a Holothuria in the water this crown of tentacles ceases to appear, for the animal has the power of withdrawing it quite suddenly, and now it resembles nothing so much as a common leech. If, however, it is preserved in fresh sea-water and left in peace--if we treat it, in short, with the regard due to its elegant crown of tentacula--this elegant ornament will be expanded in all its glory. Immediately below the mouth is a muscular pharynx, which is contained in a long intestine, with many convolutions, which terminate in the posterior part of the body in an orifice whence is thrown from time to time a little jet of water. The terminal portion of the intestinal canal in these animals is enlarged, introducing us to a system of numerous tubes which branch off into the visceral cavity, receiving the water from without while breathing by its posterior extremity; the animal can at will fill this reservoir or eject the water, and it is by these alternate movements of aspiration and its reverse that it renews the oxygen necessary for respiration. The circulation appears to form a complete circle, there being no heart or central agent; but a ring round the gullet, from which issue five principal nervous chords, represents the nervous system.

The Holothurias are of separate sexes, and they differ from the sea-urchins and asterias in this: that their larvæ are converted bodily into a young Holothuria without losing their organs. The bodies of certain species are lubricated by an acrid and corrosive liquid: thus _H. oceania_, described by Lesson, which is about forty inches in length, secretes at the surface of its body an irritating fluid, which produces an intolerable itching in the finger which touches it. Nor can the inhabitants of the South Sea Islands look at it without loathing. Fig. 119 represents _H. lutea_, or the _Stychopus luteus_ of Brandt, who describes as its distinctive character three rows of tentacular feet on the ventral surface.

We have spoken of the strange suicidal tendency of the sea-stars: the Holothuria exhibits the same phenomena, but, having no brittle envelope like the asterias, it cannot break itself into bits in the presence of its disconcerted enemy; but kills itself in this manner: having some cause of grief and trouble--such, for instance, as the attack of an enemy or the pursuit of some fisherman--by a sudden and unexpected movement it ejects its teeth, its stomach, its digestive apparatus, and reduces itself to a simple empty membranous sac, with an unfurnished mouth; and, as a singular fact, this empty sac still shrinks and contracts in the hand which grasps it. It must be admitted that this is a strange mode of evading its enemies: the soldier rarely throws his arms away in the moment of danger! But the Holothurias possess a wonderful recuperative power also; and it is probably quite conscious, when it thus empties itself to disappoint its pursuer, that it can promptly replace the organs which it has voluntarily parted with.

Dr. Johnston relates that he had forgotten for some days to supply a Holothuria with a change of water. The creature, in consequence, ejected its tentacles, its buccal apparatus, digestive tubes, and a portion of its ovaries. Still it was not dead, but was sensible to the least movement, and lived to reproduce all its organs anew.

Not only do the Holothurias eject their organs and afterwards renew them, but they divide themselves spontaneously into two portions. Their two extremities are first enlarged; then their middle parts gradually become straight, like a thread: finally, this thread breaks, and each separate part of the animal becomes a perfect Holothuria. It has been cut into two pieces, and each of these species becomes a new being.

The habits of these animals are but little known. They inhabit the seas, and are spread over every latitude. Their very limited movements consist in a kind of reptation or crawling motion, produced by the undulations of their bodies or by the contractions of their feet. Holothurias are generally found in the act of creeping upon stones or on portions of submarine rock, but always in sheltered places, for they appear to dread the action of light. They sometimes find themselves caught by fishermen in their nets. If held in the hand they contract, their bodies become hard and rigid, and the sea water with which they are filled is ejected with force. We need not add that fishermen reject with disdain the Holothurias taken in their nets; the sea-cucumber has never been thought worthy of a place on our tables. Truth is on this side, error on that, is a maxim as true in morals as in cookery. The sea-cucumber, which Europeans disdain, is a favourite dish among the Chinese. The fishery, preparation of, and transport of these animals to market, plays an important part in the commerce and industry of the East. One rather large species, the _Holothuria tubulosa_, in which, by-the-bye, a singular parasite fish (_Fierasfer fontanesii_) lives, is common in the Mediterranean. This species is eatable, and much relished at Naples. In the Ladrone Islands _Holothuria guamensis_ is preferred. But nowhere is it esteemed of such importance as in the Malayan and Chinese seas. In these countries, and on most of the shores of the Indian Ocean, the _Holothuria edulis_, vulgarly called _Trepang_, is eaten with delight. Thousands of junks are annually equipped for the Trepang fisheries. The Malay fishermen carry to this fishery a degree of patience and dexterity truly remarkable. Lying down in the fore part of their vessels, and holding in their hands a long bamboo, terminating in a sharp hook, their eyes, accustomed to this fishing, frequently discover the animal at a distance of not less than thirty yards, as it creeps along the surface of the submarine rocks or corals. The fisher darts his harpoon at this distance, and seldom misses his prey. When the water is shallow, that is to say, not more than four or five fathoms deep, divers are sent down to obtain these culinary monsters, who seize them in their hands, and in this manner can take five or six at a time. To prepare the fish and preserve them for transport to the markets, the Malay and Chinese fishermen boil them in water, and flatten them with stones. They are then spread out on bamboo mats to dry; first in the sun, and then by smoking them. Thus prepared, they are enclosed in sacks, and shipped to the Chinese ports, where they are particularly esteemed. This fishery takes place in the months of April and May.

In his voyage to the South Pole, Captain Dumont d'Urville, in traversing the Chinese seas, had an opportunity of assisting at this fishery, which he has described very graphically. We quote the passage in which the French navigator relates what he witnessed at this curious scene. While the ships were lying quietly at anchor, "we saw," he says, "entering the bay, four Malay proas, bearing Dutch colours, which dropped their anchors about a cable's length from Observatory Islet. The padrones or captains of these vessels soon presented their salutations, and informed me that they had started from Macassar at the end of October, with the western monsoon, and that they came to fish for Holothuria (trepang) along the coasts of New Holland, from Melville Island to the Gulf of Carpentaria, where the east wind met them and assisted their return, when they revisited all the points of the coast, anchoring in every bay where they hoped to find fish. We were in the first days of April; the east monsoon was definitively established; the Malay fishermen were returning in their circuit, and, in passing, they came to exercise their industry in Raffles' Bay. An hour after their arrival they were all at work, and the laboratory for the preparation of their fish was established within our view. The roadstead had no longer the aspect of a vast solitude: wreaths of smoke crowned the summit of Observatory Island, where, as if by enchantment, several large sheds had sprung up, while numerous vessels, supplied with divers, were proceeding to fish for Holothurias, which were passed immediately to the furnaces erected for curing them. In the course of my voyage I have often remarked little walls constructed of dry stones, consisting of several half-circles joined one to the other. I had often, but vainly, tried to discover the use of these little structures: I was now enlightened. The Malays arrived. Their boats were scarcely anchored when several large boilers, in the shape of a half-sphere, the diameter of which might be about forty inches, were placed upon the stone walls of which I have spoken, and now served as improvised furnaces. Near to them are sheds, composed of four strong posts driven into the earth, supporting roofing covered with hurdles, on which it is probably intended to dry the Holothurias. During their sojourn in this bay, the fishermen, having fine weather, made no use of these sheds, having probably only prepared them as a precaution.

"A crowd of men actively employed in establishing their laboratories gave an unaccustomed appearance to the bay, which could not fail to attract the savage inhabitants of the main land. Very soon, indeed, we could see them hastening from all sides, and nearly all reached the little island, either by swimming or wading through the sheet of shallow water which separates it from the main land. I only saw one pirogue, made of the bark of a tree badly put together, which gave a passage to three of these visitors. When night arrived, the Malays had finished all their preparations; some of them remained to guard what they had left on shore, all the others returned to their boats.

"In the interval, a boat from the Astrolabe being wanted to carry some visitors from the island, I profited by the occasion to visit one of the proas, accompanied by M. Roquemauel. We were received with much politeness, and even cordiality, by the captain or padrone of the boats. He showed us over his little ship. The keel appeared to us sufficiently solid; even the lines did not want elegance; but great disorder seemed to reign in the stowage department. From a kind of bridge, formed by hurdles of bamboos and junk, we saw the cabin, which looked like a poultry-house; bags of rice, packets, and boxes were huddled together. Below was the store of water, of cured trepang, and the sailors' berths. Each boat was furnished with two rudders, one at each end, which lifted itself when the boat touched the bottom. The craft was furnished with two masts, without shrouds, which could be lowered on to the bridge at will by means of a hinge; they carry the ordinary sail; the anchors are of wood, for iron is rarely used by the Malays; their cables are made of ratan fibre; the crew of each bark consists of about thirty-seven, each shore-boat having a crew of six men. At the moment of our visit they were all occupied in fishing operations, some of them being anchored very near to us. Seven or eight of their number, nearly naked, were diving for trepang; the padrone alone was unoccupied. An ardent sun darted his rays upon their heads without appearing to incommode them, an exposure which no European could hold up under. It was near mid-day, and the moment, as our Malay captain assured us, most favourable for the fishing. In fact, we saw that each diver returned to the surface with at least one animal, and sometimes two, in his hands. It appears that the higher the sun is above the horizon, the more easily is the creature distinguished at the bottom. The divers were so rapid in their movements, that they scarcely touched the boat, into which they threw the animals, before they dived again. When the boat was filled with them, it proceeded to the shore, and its place was supplied by an empty one. I followed one of these, to witness the process of curing which they adopted.

"The Holothuria of Raffles' Bay is from five to six inches long and about two in diameter; it is a gross fleshy mass, somewhat cylindrical in form, but no external organ is visible. The mollusc glues itself to the rocks at the bottom of the sea, and, as it can only move very slowly, the Malay divers seize it readily. The greatest merit of a fisherman is to have a practised eye, to distinguish the animal at the bottom, and to dive directly to the spot where it lies. To preserve them, the fishermen throw them, while still living, into a cauldron of boiling sea water, where they are stirred about by means of a long pole, which is supported upon another pole fixed in the earth, but having a forked end, which acts as a lever. In this process the trepang gives up all the water it contains, and is withdrawn at the end of two minutes. A man armed with a large knife now extracts the entrails, and it is thrown into a second cauldron, having only a small quantity of water, seasoned with mimosa bark. The object of this second operation is to smoke the animal in order to preserve it the better, for the bark is consumed in the process. The trepang is now placed upon hurdles and dried in the sun. When sufficiently dried, it is stowed away in the hold of the proa.

"It was about two o'clock in the afternoon when the divers ceased their labours and came ashore. My tent was soon surrounded. I recognized the captain of the proa among those who had previously visited me. He approached and examined all the instruments used in the Observatory with great attention, seeking to discover their use. I showed him a gun with percussion cap, which astonished him greatly, especially when I pointed out to him its great superiority over the flint-lock. He assured me that these arms were still unknown in the Celebes, his country; but he failed to convince me of that. He questioned me as to the places we had visited, and where we were going. I endeavoured to sketch a map of New Holland, New Zealand, and New Guinea upon a leaf. He then took my pencil, and added to it the Indian Archipelago, the coasts of China and Japan, and the Philippine Islands. Surprised in my turn, I asked him if he had visited all these places. He replied in the negative; but added that he knew their position perfectly, and could easily take his vessel to any of them. Finally, the interview terminated by his asking for a glass of arrack. I do not know if this intelligent Malay professed the Mahometan religion, but I do know that he drank half a bottle of wine and a quarter of a pint of arrack without being at all the worse for it. He then offered me some prepared trepang, inviting me to taste it, which I did; to me it appeared to resemble the lobster in taste. My men liked it, and thankfully accepted the captain's offer; for my part, I felt an utter repugnance even to taste it.

"According to the account I had from the Malay captain, the price of trepang in the Chinese markets was fifteen rupees, about thirty shillings the pekoul, or a hundred and twenty-five pounds. He estimated his cargo to be worth about a hundred and twenty pounds. The fishing had occupied him and his crew three months. From the earliest times this commerce has belonged exclusively to the Malay fishermen, and it will always be difficult for Europeans to compete with them. The Malay vessels are equipped on the most economical principle, and the men are wanting neither in sobriety, intelligence, or activity.

"It was nearly four o'clock when the Malays finished their operations. In less than half an hour they had embarked their cargo; the tents were struck, and, together with the boilers, carried back to the boats, which were already preparing to set sail. At eight o'clock in the evening they hoisted sail and left the bay."

Some idea may be formed of the extent and importance of the Holothuria fishing by the number of ships which it attracts in this part of the East. Captain King assures us that two hundred vessels annually leave Madagascar to fish for the _sea slug_, as it is sometimes called. Captain Flinders, being on the coast of Australia, learnt that a fleet of sixty vessels, having a hundred men on board, had left Madagascar two months previously in the same pursuit.

Among the Holothurias, one particular genus, the _Synapta_, is distinguished from others of the family by the absence of the ambulacral feet, and by the fact of its uniting both sexes in one individual. This remarkable Echinoderm, _Synapta duvernæa_, is represented in PL. XI. M. Quatrefages, who discovered it in the Channel, gives the following description of it in his great work, "Le Souvenirs d'un Naturaliste." "Imagine," he says, "a cylinder of rose-coloured crystal, as much as eighteen inches long and more than an inch in diameter, traversed in all its length by five narrow ribbons of white silk, and its head surmounted by a living flower, whose twelve tentacles of purest white fall behind in a graceful curve. In the centre of these tissues, which rival in their delicacy the most refined products of the loom, imagine an intestine of the thinnest gauze gorged from one end to the other with coarse grains of granite, the rugged points and sharp edge of which are perfectly perceptible to the naked eye.

"But what most struck me at first in this animal was, that it seemed literally to have no other nourishment than the coarse sand by which it was surrounded. And then when, armed with scalpel and microscope, I ascertained something of its organisation, what unheard-of marvels were revealed! In this body, the walls of which scarcely reach the sixteenth part of an inch in thickness, I could distinguish seven distinct layers of tissue, with a skin, muscles, and membranes. Upon the petaloid tentacles I could trace terminal suckers, which enabled the Synapta to crawl up the side of a most highly polished vase. In short, this creature, denuded to all appearance of every means of attack or defence, showed itself to be protected by a species of mosaic, formed of small calcareous shield-like defences, bristling with double hooks, the points of which, dentated like the arrows of the Caribbeans, had taken hold of my hands."

If one of these Synapta is preserved alive in sea-water for a short time, and subjected to a forced fast, a very strange phenomenon will be observed. The animal, being unable to feed itself, successively detaches various parts of its own body, which it amputates spontaneously. A great compression or ring is first formed, and then the separation of the condemned part takes place quite suddenly. "It would appear," says M. Quatrefages, "that the animal, feeling that it had not sufficient food to support its whole body, was able successively to abridge its dimensions, by suppressing the parts which it would be most difficult to support, just as we should dismiss the most useless mouths from a besieged city."

This singular mode of meeting a famine is employed by the Synapta up to the last moment. After a few days, in fact, all that remains of the animal is a round ball, surmounted by its tentacles. In order to preserve life in the head, the animal has sacrificed all the other parts of its body.

In order to find natural novelties--to find unforeseen subjects of study and reflection, it is not necessary to run over the world or travel great distances. It is only necessary to visit the banks of the nearest river, or descend to the sea shore, and leave the sea to reveal a fragment of the marvels which it conceals in its bosom.

MOLLUSCA.

The class Mollusca--pulpy animals--forms a grand division which man has been pleased to make in the animal kingdom, and immediately below the Vertebrata and above the Annulosa, which again stand above the Coelenterata, which includes the polyps, sea-anemones, hydras, and corals, which last are more highly organized than the Protozoa.

The Mollusca may be divided into two groups, the Mollusca proper and the Molluscoïda. The mollusc proper, as represented in Fig. 120, presents the following parts, and is supposed to be bilaterally symmetrical, H, is the hæmal parts, in which the heart is situated, commonly called the dorsal part, although the word is used in a different sense in different divisions of the animal kingdom. In the same manner the opposite region (N) is not termed the ventral, but the neural part, in philosophical anatomy. It is the region in which the great centres of the nervous system are placed. The termination (_a_) is the anterior or oval part; the other end (_b_), the posterior or anal part: between these extremities the intestines take a straight course. The neural surface is that upon which the majority of molluscs move, and by which they are supported, and it is commonly modified to subserve these purposes by the formation of a muscular expansion or disk, called the foot. Three regions, in many genera very distinctly divided from one another, may be distinguished in this foot: an anterior, the _Propodium_ (_p p_); a middle, the _Mesopodium_ (_m s_); and a posterior, the _Metapodium_ (_m t_). In addition to these, the upper part of the foot, or middle portion of the body, may be prolonged into a muscular enlargement on each side, just below the junction of the hæmal with the neural region, the _Epipodium_ (_e p_). The mass of the body between the foot proper and the part of the abdomen which bears the epipodium may be termed the mid-body, or _Mesosoma_. On the upper part of the sides of the head are two pairs of organs, namely, the eyes and tentacles. In the hæmal region the integument may be modified and raised up into a fold at the edges, either in front or behind the anus. When so modified, it is called a mantle, _Pallium_. In front of the anus again, the _branchiæ_ (_t_) project as processes of the hæmal region. Among the internal organs, the heart (_u v_) lies in front of the branchiæ in the hæmal regions, the nervous ganglia (_x_ _y_ _z_), of which there are three principal pairs, being arranged around the alimentary canal, which they encircle.

Such is the general type of the class Mollusca, of which, however, the variations are innumerable. They are all soft-skinned animals, without either articulated exterior or annular external skeleton. Their nervous system, being without cerebro-spinal axis, is entirely composed of ganglions, which are all reunited in the oesophagus without constituting in any case a lengthened median chain. Their digestive organs are complete--that is, they are provided with two apertures; their principal organs are symmetrical and according to a plan, usually curving, by which their bodies are divided into two parts.

The first series or subdivision, to which Milne Edwards has given the name of _Molluscoïda_, includes under that term the Bryozoaires, Ascidians, and Tunicata.