CHAPTER X.

MOLLUSCOÏDA.

The Bryozoaires, or Polyzoa, as British naturalists prefer to call them, form the boundary-line which divides the humble mollusc from the humbler zoophytes. In consequence of this intermediate organization, these creatures were long considered as polyps; but De Blainville, Milne Edwards, and Ehrenberg, almost simultaneously began to separate them from the molluscs, and form them into a separate group. Subsequent naturalists, while considering the Molluscoïda as truly and wholly molluscous, admit that the distinction proposed by the French naturalists is most important, and should be retained as a primary subdivision, confining it to those molluscs which have the neural region comparatively little developed, and the nervous system reduced to a single or at most a pair of ganglia, and the mouth surrounded by a more or less perfect circle of tentacles: an arrangement which would include the _Brachiopoda_ with the _Polyzoa_.

Marine plants are sometimes observed to be quite covered with a velvety parasitic matter, which may at a first glance be mistaken for a moss. This, however, is simply an aggregation of animalcules, each of which has its separate cell, which is placed quite contiguous to its neighbour.

These little creatures are thus entirely distinct. Each cell is formed by the skin, which has been encrusted by calcareous salts, or other organic matter, hardened after the manner of a horn. This kind of shell protects the animal from the attacks of its enemies. This mode of retreat at the bottom of a protecting shelter is very frequently adopted in the whole series of molluscs. The oyster shuts itself up by closing its valves, and the snail retires into its shell. This assemblage of small cells presented by the Bryozoaires has long been known as a coral. "We propose," says our author, "with very good reasons, to call it a _Testier_, or shell-builder."

This testier, in which each shell has its opening, is furnished with a naked cushion, dentate, spinous, or protected by an operculum or lid, and presents itself under every variety of form. It is sometimes an assemblage of branching tubes, occasionally a rounded mass of spongy appearance, and now it presents itself as a flat lamelliform inarticulated expansion. In some of the marine species the shell of the mussel is covered as with a fine lace.

It is a remarkable fact that these cells are not always inert. They seem to enjoy the power of motion. It is well known that the leaves and branches of the sensitive plant (_Mimosa_) contract and expand under the touch of the finger; the same phenomenon, according to Mr. Rymer Jones, takes place on touching the cells of certain species of Bryozoaires. The moment they are touched they quickly incline themselves; and the movement is immediately communicated from one to the other, until all the cells of the community are in motion.

Returning to the organization of the little creature which occupies the cell, it is found that the upper and retractile portion, which is of extreme delicacy, terminates anteriorly in a circle of long tentacles, in the centre of which is the mouth. These tentacles are fringed laterally by a series of vibratile cilia. "When the animal displays itself," says Frédol, "this circle of microscopic threads of extreme tenuity first show themselves rising from the summit of the cell; this is followed by the upper part of its body, which is more or less flexible; the tentacles follow between the threads, pushing them on one side."

These tentacles are furnished on the back with a dozen appendages like very fine hairs, attached to them nearly at right angles, in addition to the lateral cilia already spoken of, which play a very important part in the arrangements of most microscopic animals. At the moment when the tentacles appear outside the cell, the tunic of the animalcule, which has the power of expanding or contracting itself, is gradually unrolled. It soon spreads out its pretty little arms, the appendages and cilia beginning their rapid vibrations, until the eye, deceived by the rapidity and regularity of their movements, is dazzled, and the beholder begins to think that he sees rosy drops of dew waving to and fro, twisting and untwisting themselves. The corpuscles which float round the animal are violently agitated, as if they were under the influence of some strong breeze. Unhappy, indeed, is the fate of the unfortunate infusoria which chance leads at this moment into the fatal circle.

In many species, observers have discovered a special organ called the _vibracule_, which deserves our attention for a moment. It is a hollow filament, situated at the upper and outer angle of each cell, filled with a substance which is at once fibrous and contractile, admitting of some very remarkable movements, which occur regularly, and generally at very short intervals. At first the filament inclines itself towards the base, trembles, oscillates, and seems to sink; presently it recovers itself, and inclines in the opposite direction, where it repeats the same operation with the same order and in the same time. "What are the functions thus performed?" asks Frédol. "Are they, we would ask, independent up to a certain point of the will of the Bryozoaire? What is their purpose?" We think he answers, "That this organ serves the purpose of cleansing, and especially that of strengthening, the entrance to the cell. It even continues its movement after the animal has been mutilated or killed. The poor sickly or dead creature continues to be defended by its protecting vibracule."

The prey which is drawn into the vortex by the tentacles and their appendages enters into the mouth, to which is attached a pharynx, oesophagus, stomach, and intestines. In the back or hæmal region, not far from the mouth, there is a special opening for this intestine.

Respiration is provided for in the Bryozoaires by the ciliate appendages which surround the mouth; they are at once tentacula and branchiæ. The animal presents no other trace of organs of the senses. A small ganglion and a few fillets constitute all of the nervous system which can be traced; neither heart nor blood-vessels have been found.

The egg, in the case of the Bryozoaires, gives birth to a young animal covered with hairs on its surface; it swims about freely until it has chosen a convenient place in which it can establish the new colony which it is to originate. But this choice is not made for itself alone; the young animal encloses under its hairy envelope two new individuals, which, young as they are, have already the appearance of adult Bryozoaires. At first, these only increase the number of the colony by budding, but in a short time they produce eggs.

From these remarks it will be seen that the animals of the Bryozoaires are more complex in their form and functions than those of the coral, and the study of their anatomy confirms this conclusion. In their case the digestive organs are no longer a simple sac with a single orifice; there is a mouth, a pharynx, a gullet, a gizzard, a membranous stomach and intestines, with a special opening. We have descriptions of some species in which the gizzard seems to be provided with a certain number of interior teeth forming a wonderful pavement--a living mill for the purpose of grinding the food before it enters into the second stomach. The organization of this small creature reveals to our eyes a wonderful amount of combination--of admirable art immeasurably surpassing all that the most perfect human industry and human genius can accomplish.

After this general view of the organization of the group, we shall proceed to introduce the reader to some of their more characteristic species.

Under the leaves of water-lilies (_Nymphea_), pond-weed (_Potamogeton_), or upon floating fragments of submerged wood, are generally to be found certain Bryozoaires, animals described by Trembley under the name of _plumed polyps_. These are _Plumatellæ_ (Fig. 121). These little diaphanous creatures constitute colonies which under the microscope resemble small branching shrubs; they consist of small slender tubes grafted one to the other, and having from forty to sixty retractile tentacula, which expand like the petals of a flower; they are furnished with vibratile cilia, the movements of which serve the purpose of leading food into the mouth.

Another genus, which is found in ponds in France, and which is also found in fresh water in Britain, is the _Cristatella_ of Cuvier. "Perfect specimens of _C. mucedo_ occur from six lines to twenty-four in length by two or three in breadth," says Sir J. G. Dalyel, "of a flattened figure, fine translucent green colour, and fleshy consistence. Some of the shorter tend to an elliptical form, but those of larger dimensions are linear, with parallel sides, and curved extremities. The middle of the upper and the whole of the under surface are smooth, the former somewhat convex, occasioned by a border of seventy or eighty, even up to three hundred and fifty, individual polypi, dispersed in a triple row, their number depending entirely on the size of the specimen. Each of these numerous polypi, though an integral portion of the common mass, is a distinct animal, endowed with separate action and sensation. The body rising about a line above a tubular fleshy stem, is crowned by a head, which may be circumscribed by a circle as much in diameter, of a horse-shoe shape, and bordered by a hundred tentacula. Towards one side, the mouth, of singular mechanism, seems to have projecting lips and to open as a valve, which folds up within, conveying the particles which are absorbed to the wide orifice of an intestinal organ, which descends, perhaps, in a convolution below; and returns again, terminating in an excretory canal under the site of the tentacula."

The inhabitants of the colony are then united in great numbers under one common envelope; these are longish filaments of the size of a swan's feather, reminding one of the appearance of the silk thread known by embroiderers as chenille. The downy appearance is produced by the collection of tentacula belonging to this curious swarm. The filamentous mass is the translucent row of cells in which these animalcules are lodged, and to which they retreat when disturbed. These cells are sometimes free in part, sometimes completely rooted to the stems of aquatic plants. The tentacles are of a fine transparent glass colour, the body being of a brown colour. Fig. 122 represents _Cristatella mucedo_, which is common both in this country and in France.

Most naturalists have now agreed to place among the Bryozoa certain species of animalcules which long remained imperfectly known. Amongst these are the _Flustra_, the _Eschara_, and other ascidians.

The Flustra are marine Bryozoa, whose skin in hardening forms a thin shell of horny or cellular appearance; their little cells, more or less horny, are grouped symmetrically, somewhat like the cells in a bee-hive. Sometimes they form a crust which covers the algæ and other submarine bodies; sometimes they form ribbon-like stems. In some species the cells are only found on one side; in others they occupy both. Their orifices are extremely small, and defended by spines quite microscopic (Fig. 123).

Their tentacles are covered with cilia, always vibratile, disposed in a straight line, which in their movements produce the effect which a row of animated pearls might be supposed to produce if rolled upwards from the base to the summit of the organ.

The Eschara form leaf-like expansions, the entrance to their cells having also their protecting spines.

The expansions still represent microscopic bee-hives, the inhabitants of which enjoy at once a common and an independent existence. As it is with the corals, so it is here; each eats for the benefit of itself and for the community. Labour and nutrition for the community, labour and food for itself.

TUNICATA.

On seeing one of the _Tunicata_ for the first time, a stranger to zoology would scarcely take them for animals at all. Almost always attached to submarine rocks, these beings have the form of a simple sac. Their skin, gelatinous, horny, or rock-like, is at times covered with marine plants and polyps. They have neither arms, nor feet, nor head. But then they have a mouth, placed at the entrance of a digestive tube, and, in connection with the latter, a special opening intended for evacuations. The mouth is preceded by a great cavity, the walls of which are covered with vessels; for this cavity is the seat of respiration, and is covered with vibratile cilia. Thus the same canal serves first for respiration, and then, farther on, for digestion: another instance of the economy of Nature. Another remarkable instance of circulation is found: they have a heart, but no head.

This heart is the centre of a well-developed vascular system, but very unlike what usually obtains. The blood which traverses it takes such a course, that, in the space of a very few minutes, the heart changes its aurical into ventrical and its ventrical into aurical blood. At the same time the arteries are changed into veins and the veins into arteries. The consequence is, that the current which traverses these canals changes its direction with each contraction of the heart.

Simple as is their organization, the Tunicata have a nervous system. It is an unique ganglion, connected with divers small fillets. The organs of sensation present themselves in a very rudimentary fashion. We find eyes, and, after very minute search, a single ear has been found. They are propagated by budding, and also from eggs. The young are subject to some very curious metamorphoses, some particulars of which will be given farther on.

The Tunicata are divided into _Ascidia_ and _Salpa_, to which some naturalists add the _Brachiopoda_.

ASCIDIANS.

The _Ascidia_, from the Greek word ἀσχιδίον, leather bottle, have, as the name indicates, the shape of a bottle or purse. The analogy becomes more evident when it is considered that these creatures are habitually filled with water, which can be expelled by very slight pressure.

The Ascidians are sometimes free, sometimes united to others in a manner more or less intimate. Hence their division into the three groups of _simple_, _social_, and _composite_ Ascidians.

Simple Ascidians attach themselves, each individual singly, to rocks and other submarine bodies, and generally at a fixed depth. _Ascidia microcosmus_ a Mediterranean species, represented in Fig. 124, may be quoted as a type of this division of Ascidians. The name of Microcosmus, or the little world, is probably given from its being inhabited by quite an animated colony of algæ and corals, which dwell upon its surface, and give it a very peculiar, but not very attractive, appearance. The flavour of these molluscoids is very strong, which does not, however, hinder the poorer dwellers on the sea shore from eating them. The genus _Phallusia_ is another type of the group. _Phallusia grossularia_ is of a reddish colour, and about the size of a currant-berry: it usually lodges itself in the oysters of certain localities. At Ostend another species, _Phallusia ampulloïdes_, is found in prodigious quantities in the oyster parks, and is parasitic on living lobsters.

Social Ascidians comprehend living _Tunicata_, connected together on a common prolongation by the roots, but free and unconnected in all other respects. _Ascidia pedunculata_ (Fig. 125) may be quoted as an example.

The Composite Ascidians are still more intimately associated together; a great number of these little beings live together in a single mass. Such are the Botryllus and the Pyrosoma.

The Botryllus is a genera the most interesting of all the groups under consideration. Only imagine from ten to twenty individuals, oval in form, more or less flattened, adhering by their dorsal surface to some submarine body, and holding on by their sides, so as to form a sort of wheel. "When we excite one of the branches," says Frédol, "a single mollusc contracts itself; when we touch the centre, they all seem to contract themselves (Cuvier). The buccal orifice is at the outer extremity of the radius; but the intestinal terminations abut on the common cavity, which occupies the centre of the wheel. Here we behold certain animals which eat separately, but which fulfil together as a community very singular functions--a kind of union and communism of which the moral world presents no prototype. With our molluscs, in place of two individuals united, we have a score. We may consider the entire star as one single animal with many mouths. But then, we have with it a luxury of organs for the function of intelligence which seeks and chooses, and parsimony of the organ of stupidity, which neither seeks nor chooses."

While the Botryllus is fixed and adherent, the Pyrosoma, on the contrary, is perfectly free. The animal colony which constitutes it floats and balances itself upon the waters, like the sea-pen or the physalia, of which we have spoken in treating of the zoophytes.

The name Pyrosoma has been given to these animals in consequence of their brilliant phosphorescent properties. According to the observations of Péron and Lesueur, nothing can exceed the brilliant and dazzling light emitted in the bosom of the ocean by these animals. From the manner in which the colonists dispose themselves, they form occasionally long trains of fire; but it is a singular fact that this phosphorescence presents the same curious characteristics that distinguish the cilia of the Beroë; namely, that the colours vary instantaneously, passing with wonderful rapidity from the most intense red to yellow, from golden colour to orange, to green, or to azure blue. Von Humboldt saw a flock of these brilliant living colonies floating by the side of his ship, and projecting circles of light having a radius of not less than twenty inches in diameter. He could see by this light the fishes which followed the ship's track, during many days, at the depth of from two to three fathoms.

Bibra, a Brazilian navigator, having caught six Pyrosoma, employed them to light up his cabin. The light produced by these little creatures was so bright, that he could read to one of his friends the description he had written of these his living torches.

Three species of Pyrosoma are known; namely, _P. elegans_, two or three inches in length, which inhabits the Mediterranean; _P. giganteum_, which is found in the same sea. It is a long bluish cylinder, bristling with tubercles, each of which is the abode of an animal, a citizen of this moving republic, and is attached to its colleagues by means of its gelatinous envelope: an alliance imposed by inexorable Nature--a forced species of socialism.

The third species, _P. atlanticum_, was discovered by Péron and Lesueur in the Equatorial seas.

These curious Ascidians are so created in rings as to constitute a long fine cylindrical tube, closed at one end and open at the other. By the contraction and dilatation of the mass of beings, this great cylinder swims slowly through the open sea, lighting up the ocean with its phosphorescent light, shining through the water like a glowing fire. Mr. Bennet thus describes one of these pelagic appearances: "On the 8th of June, being then in lat. 30° S. and 27° 5' W. long., having fine weather and a fresh south-easterly trade-wind, and the thermometer ranging from 78° to 84°, late at night the mate of the watch called me to witness a very unusual appearance in the water. This was a broad and extensive sheet of phosphorescence extending from east to west as far as the eye could reach. I immediately cast the towing-net over the stern of the ship, which soon cleaved through the brilliant mass, the disturbance causing strong flashes of light to be emitted, and the shoal, judging from the time the vessel took in passing through the mass, may have been a mile in breadth. On taking in the towing-net, it was found half filled with _Pyrosoma atlanticum_, which shone with a beautiful pale greenish light. After the mass had been passed through by the ship, the light was still seen astern, until it became invisible in the distance, and the ocean became hidden in the darkness as before this took place.

"The second occasion of my meeting these creatures was in a high latitude, and during the winter season. It was on the 19th of August, the weather dark and gloomy, with light breezes from north-northeast, in lat. 40° 30' S., and 138° 3' E. long., at the western entrance to Bass's Straits, and about 8 o'clock p.m., when the ship's wake was perceived to be luminous, while scintillations of the same light were abundant all round. To ascertain the cause, I threw the towing-net overboard, and in twenty minutes succeeded in capturing several Pyrosoma, which gave out their usual pale green light; and it was, no doubt, detached groups of these animals which were the occasion of the light in question. The beautiful light given out by these molluscans soon ceased to be seen; but by moving them about it could be reproduced for some length of time after. The luminosity of the water gradually decreased during the night, and toward morning was no longer seen."

The genus _Salpa_ forms another interesting group of Tunicata.

The Biphora or Salpa (Fig. 126) are long transparent threads of the more delicate tissues, composed of rows of individuals placed side by side, and grafted, as it were, transversely: ribbons, in which each animal is grafted end on end to its sister: double parallel chains of social creatures, sometimes alternate, sometimes opposite; living chaplets, of which each pearl is an individual. Each individual presents an oblong diaphanous or prismatic body, more or less symmetrical, and often furnished in front, rarely behind, with tentaculiform appendages. So great is the transparency, that the various organs may be observed through the skin as they perform their several functions.

Momus, an ancient philosopher, thought it a subject of regret that Nature had not thought of piercing the body with an opening sufficiently large for each one to see what was passing in the interior. The creature which now occupies our attention would surely have satisfied the demands of our critic: its body is, metaphorically speaking, a house of glass.

In order to move itself, the Salpa has recourse to a singular artifice. It introduces water into its body through a posterior opening, furnished with a valve, which it expels by an anterior outlet situated near the mouth. It is thus pushed backwards, and swims, as it were, by recoil. Moreover, it swims with its belly upwards. All the elements of a chain of Salpas act in concert; they contract and dilate simultaneously; they advance as a single individual. One of them floats on the surface with the undulations of a serpent, so that among sailors they have gained the appellation of sea-serpents. These long, living trains abound in the Mediterranean, principally towards the African coast, and in the Equatorial seas. They are inhabitants of the open sea, and live immerged at considerable depths; but when the nights are calm they show themselves on the surface. As they spread themselves abroad with a strong phosphorescent light, they resemble long ribbons of fire, unrolling their long waving lines in spite of the waves, as in Fig. 127. What wonders they see who go down into the great deep! What sights are reserved for the navigator who traverses the Tropical seas during the silence of night!

When a chain of Salpas is drawn from the water, the rings separate, and they can no longer be made to adhere. The social bond has been dissolved by a superior force.

Salpas are sometimes met with, isolated and solitary, whose exterior conformation differs much from that which is proper to the connected Salpa; so different, indeed, that it might belong to another type. Chamisso, Krohn, and Milne Edwards have ascertained that the Biphora is viviparous, and that each species is propogated by alternate generation, the young creature being unlike its immediate parent. One of these generations is represented by the solitary individuals, the other by an aggregation of individuals. Each solitary Biphora engenders a new group--a chain; each constituted member of the chain engenders a solitary Salpa.

Thus a Salpa is not organized like its mother or daughter, but rather like its sister, its grandmother, or granddaughter--another example of alternate generation, which has already been discussed in treating of zoophytes.

Those marine creatures which pass their lives in a forced community--animals which eat, sleep, or rest always in company--who abandon themselves together to the soft caresses of the waves,--these colonies, or, rather, republics of animals, leading constantly the same monotonous existence,--reveal to us very strange things: an identical community of sentiments in a crowd of beings riveted by the same chain--a chain at once physical, intellectual, and moral!