CHAPTER V.

Functions and Instincts. Polypes.

THE tribe of animals to which we are next to direct our attention, though not invisible like the last, are almost equally concealed from our view by the medium that they inhabit; so that, with the exception of those that abound in fresh water, and are easily kept alive for examination, the great body of them inhabiting the ocean, can seldom be studied in a living state. All the polypes are aggregate animals, in which they differ from the majority of the preceding class. The most imperfect of them, as the sponges and some of the alcyons, seem to consist merely of a gelatinous mass, without any organs of prehension, which by its alternate contraction and dilatation, imbibes or sends out the water from which the animal derives its nutriment; but the great majority have a mouth furnished with arms or tentacles varying in number. These are described as tubes, filled with fluid, expanding at the base into a small cavity, which when contracted necessarily propels the fluid into the tentacles, and thus extends them; but when the tube contracts, the fluid flows back into the cavity, and the points of the tentacles converge over the mouth.

These parts are not only organs of sense, but also serve many other purposes, particularly those of prehension and motion; and they very probably assist in respiration, which appears evidently connected with the alternate contraction and expansion of these animals. They are also so constructed as to lay hold of every substance that floats within their reach, whether by means of any gummy excretion like bird-lime, as some suppose, or whether they are furnished with very minute suckers by which they can adhere to any substance, has not been ascertained. Trembley observed, that when the common polype of fresh water touched any little animal with one of its long tentacular arms, it was immediately arrested, and in spite of the most violent efforts to liberate itself, which he compares to those of a fish that had been hooked, was held fast, and carried to the mouth of the polype and swallowed.

The body of polypes is formed of a kind of inspissated mucus,

with confusedly agglomerated, and probably nervous molecules equally distributed; it is covered by no skin, is extremely contractile, and forms an alimentary sac open at one end, serving both for mouth and anal passage. The equal distribution of nervous molecules through the whole substance of these animals, will account for their extreme tenacity of life. In fact, this uniform gelatinous mass, which is without any organized structure, may be regarded as a kind of primary substance, which possesses characters, in some respects, common to both animal and vegetable matter.

This substance without any nervous centre-though nervous influence, one would think, must be in most force round the orifice where the tentacles are in action,-yet full of cerebral matter, sensible to the light without any organ of sight; extremely irritable; alternately contracting and expanding, and thus moving without any apparatus of muscles; with no trace of organization but the tubular rays that surround its mouth, which appear to perform the office of eyes, hands, feet, and lungs; this singular substance lends a clew to form the class into Orders according to the circumstances in which it is placed. 1. In the common Polypes' of our ditches and stagnant waters, it is a naked branching elementary sac or canal, without any internal support, and endued with powers of locomotion. 2. In the Madrepores and others, its Maker for mighty purposes has enabled the animal to form for itself a fixed calcareous house or polypary as it is called, consisting often of innumerable cells, each containing a separate individual with its mouth and tentacles, united to the general body at its other extremity, and each with an external aperture, by which they are protruded, and expand like a flower.

3. In the Coral and affinities, it forms an internal calcareous axis, which it envelops as the bark does the tree it is fixed by its base like the preceding tribe; and from this crust, or bark, the tentaculiferous mouths of the polypes emerge. In some the axis appears articulated.

N.B. In these two last the base by which the compound animal is fixed to rocks, or other substances, expands like the base or root of a tree; and, by their ramifications, these poypes, whether the polypary is external or internal, resembles its branching stem.

4. The Sponges and Alcyons have been generally arranged with the last Order, but, from M. Savigny's observations, it

1 Hydra viridis fusca, &c. 3 Corticifera, Lam.

2 Lamellifera, Lam.

4 Spongia.

5 Alcyonium.

appears that certain of these animals have neither stomach, mouth, nor tentacles, the animal life of which he thinks might be disputed; but Mr. Bell has discovered that they alternately imbibe and expel that fluid, which seems to prove their animal nature. Perhaps they ought to be considered as nearer to vegetable matter than the other polypes.

5. Other Alcyons1 seem to have a more complex organization than any of the preceding polypes; they are stated to have eight parallel stomachs. Only four genera belonging to this Order have been described, and its proper station seems doubtful.

6. In the Sea-Pen, and others, the animal envelops an axis, as in the third Order, and has a tentacular mouth, but it is not fixed by its base. The greater part of these animals float in the waters, but others remain at the bottom, either upon the surface or partly plunged in the sand.

Polypes are invariably aquatic animals, some inhabiting fresh water, but the great body are marine, and most nume rous in tropical seas. In very high latitudes, only cellarians,1 sertularians, alcyons, and some sponges occur, and in the vicinity of volcanic islands in the Polar seas, corallines and gorgonians. These multiply a little from 6° to 9° N. L.: then, as they approach the tropics, the coral reddens, and the madrepores whiten, and at 33° they attain their full powers of growth and multiplication. Some frequent the mouths of rivers, where there is a conflux of fresh and salt water. Some love atmospheric influence, while others avoid it. The marine ones frequently plant themselves on rocks, in different aspects, often regulated by the climate. They rarely expose themselves to violent currents, or the direct shock of the waves. They are often found in the hollows of rocks or submarine grottoes, and in gulfs where the water is less agitated.

It was observed above, that the Infusories present some analogy to the seeds of vegetables; the polypes go farther, and cpresent, often most exactly, the developed plant from the tree, by almost all the intermediate stages, to the fungus, at least the fixed polypes: these appear, as it were, to take root, to send forth branches which produce seeming blossoms, com posed of what appear to be petals arising from a calyx, arranged sometimes in a single and at others in a double circle, and in some including the semblance of stamina; they are also very sensible to the light, and turn to its source, and like plants

1 Polypi tubiferi, Lam, 4 Sertularia.

2 Polypi natantes, Lam.
5 PLATE II.

3 Cellaria.

are readily propagated by cuttings and buds; so that all the older naturalists regarded them as real plants, without apparently suspecting their animal nature. Ancient naturalists were very apt to mistake analogical resemblances for proofs of affinity, but in the progress of science, when natural objects were submitted to a stricter examination, more correct ideas were substituted for these mistaken ones, and the zoophytes, or polypes, were generally admitted to be real animals, though some, after Linné, still regarded them as something between animal and vegetable. Trembley was one of the first who ascertained their animal nature; he saw the fresh-water polypes,' by means of their long tentacles, seize and swallow certain grubs, and also many minute Entomostracans, common in stagnant water. These polypes so used their tentacles as evidently to indicate a degree of volition, sometimes using one and sometimes many, as circumstances required. When they had secured their prey, they contracted and gave a curve to these organs, so as to bring it near the orifice, or mouth, at their anterior extremity, which then began to open, and the anima they had caught was gradually absorbed. He has seen then attack small fishes, also worms, larvæ, and pupa of gnats, parts of slugs, entrails, and even pieces of meat.

The marine polypes are equally ravenous with the river ones, feeding upon whatever they can lay hold of, sometimes, like the wheel-animals, or rotatories, producing a vortex in the water, and thus causing a flow to their mouth of the infusory, and other animalcules contained in that element. It is to be observed that these inhabit a common house, from which they cannot separate themselves; their sole character is that of being attached to an animated mass, so that each individual partakes of the life common to the whole, and also of a separate life, independent of that of the others. Yet the nutriment that one of these individuals takes, extends its influence to parts the most distant from the place it occupies.

Having made these general remarks, I shall next give a history of some of the best known and most interesting species.

1. The common polypes of stagnant waters, belonging to the first Order, have met with an admirable historian in M. Trembley, and what I have to communicate with respect to them will be chiefly derived from him. With regard to their reproduction, it is by germs and cuttings. The former issue gradually from the body of the parent polype, as the trunk of a tree sends forth a branch. The bud that forms the commencement

1 Monoculi.

of a young one, is a continuation of her skin, and its stomach of her stomach. When she takes her food, the bodies of her young are seen also to inflate themselves as if they had taken it with their own mouths, and the food may be seen passing from one to the other. After they have grown thus as branches for some time, and even have pushed forth germs themselves, they detach themselves from the parent stem, and become separate animals.

It is stated that, by this mode of generation, in the space of a month a single polype may be the parent of a million of descendants. Trembley observed some long branches of trees that had fallen into the water, which he describes as being as full of polypes as a peruke of hairs; and that though their innumerable arms were at work, there was no confusion amongst them.

But these animals, as is well known, do not multiply solely by germs, but also by cuttings, as they may be called; their substance is so instinct with life, that nothing appears able to destroy it—a circumstance, perhaps arising from the nervous molecules of which it seems almost to consist. If divided transversely, each segment will become a distinct animal, send forth tentacles round its upper aperture, and close the lower one; if it is divided longitudinally, each half will form a separate tube in an hour, and begin to ply its tentacles in a day; even if divided into longitudinal strips, instead of the sides turning in, as in the former case, each strip becomes inflated, and a tube is formed within it: and what is still more wonderful, and seems next to a miracle, these animals may be turned inside out, like the finger of a glove, without de stroying either their vitality, their power of producing germs, and of catching, swallowing, or digesting their food; so that they have, properly speaking, neither a within nor without, both surfaces of their alimentary canal being equally fitted for di gestion. This, however, is not so entirely anomalous as it may at first sight appear; for cuttings of some vegetables, if planted inversely, will take root, the top bearing the root, and the bottom the branches and inflorescence.

The fresh-water polype usually remains fixed by its closed extremity to one spot, from which it seldom moves, exhibiting no other trace of an animated being than the motions of its arms: but when the want of light or heat causes it to shift its quarters, it moves slowly by fixing alternately, like a leech, its head and tail to what it is moving upon.

The majority of the marine polypes are attached, in some way, to a calcareous support formed by themselves, which is