POLYPI, or POLYPS, a class of animals•which were, till the last few years, included in the RAIMATA of Cuvier, but which, since the radiata have ceased to be regarded as a subkingdom, have found a place in the subkingdom CCELENTERATA. See SUBKING DOMS, ANIMAL. The name polypi, or polyps, was given by Reaumur about the-middle of the last century to these animals, on account of their external resemblance to the many-armed cuttlefishes, which were so denominated by Aristotle; and our knowledge of these organisms, as members of the animal kingdom, hardly dates back much more than a century. All polyps are aquatic in their mode of life, and almost all of them are inhabitants of the sea, two genera only (hydra and cordylophora) of fresh-water polyps being as yet known. Most of them live in societies of considerable extent, supporteil on a common stock, to which the term polypidom, (polyp-home) is usually given, and which is sometimes horny, and sometime calcareous. 'the polyps are either imbedded in cavities in the substance of the calcareous polypidom, or inclosed in minute cups or tubes, from which the body can be protruded, and into which it can be retracted at pleasure, in the horny poiypidoms. The solitary species often attain a considerable size (as. for instance, many of the actinias); but the social polyps are always minute, although the combined power of some of the species in modifying the earth's crust is neither slight nor limited in extent. "They have built up a barrier reef along the shores of New Caledonia for a length of 400 m.; and another, which runs along the n.e. coast of Australia, 1000 m. in extent. To take a small example: a single atoll (or coral island) may be 50 in. in length by 20 in breadth; so that if the ledge of coral rock forming the ring were extended in one line, it would be 120 in. in length. Assuming it to be a quar ter of a mile in breadth, and 150 ft. deep. here is a mound, compared with which the wails of Babylon, the great wall of China, and the pyramids of Egypt are but children's toys; and built, too, amid the waves of the ocean, and in defiance of the storms."— Owen, Lectures on the Invertebrate Animals, 2d edit., p. 143.
The bodies of these animals are generally soft, and cylindrical or oval in shape; and the mouth, which is the only aperture of the digestive canal, and is quite destitute of any masticating apparatus, lies in the center of the anterior or free extremity of the body, and is surrounded by a fringe or circle of tentacles or arms. The skin in the social polyps is exceedingly soft and delicate: but in the solitary species it is often of a leathery consistence. It almost always contains peculiar urticatiug organs, or thread-like cells, which may be regartied as one of the distinctive characters of the ccelenterata. Various arrangements of the polyps have been proposed, but it is sufficient for all practical pur poses if we admit two orders—namely, the hydrozoa and the anthozoa (or actinozoa), which differ essentially in the following points: in the hydrozoa the wall of the digestive sac is not separated from that of the somatic (or bodily) cavity, and the reproductive organs are external; while iu the anthozoa the wall of the digestive sac is separted from that of the somatic cavity by an intervening space, subdivided into chambers by a series of vertical partitions, on the faces of which the reproductive organs are developed. The hydra (q.v.), or fresh-water polyp, is the type of the hydrozoa. A few of these polyps are simple animals, as, for example, hydra, corymorpha, tortielara, and myriothela; but the greater number are compound or composite, exhibiting a numerous colony, connected with one another by a common trunk or camosare (from the Gr. keinoa, common, and cars, flesh), which usually presents an erect tree-like form. A sufficient idea of the form and structure of the simple pblyps of the class will be obtained by a reference to the article Ilviarta, which attains a length of between 4 and 5 inches, and was discovered by Forbes and Goodsir when dredging in the n. of Scotland. They observe that when it was placed in a vessel of sea-water, it presented the appearance ot a beautiful pink flower, its head gracefully nodding (whence the specific name given to it by Stirs, who had previously discovered it on the Norwegian coast), and bending the upper part of the stem; it waved its long white tentacles to and fro at pleasure, but seemed to have no power of contracting them. The
compound hydrozoa include, inter cdia, the orders sertularidce (embracing the various species of sertularia, campanularia, laomedea, etc.), and lubularidcs (embracing the vari ous species of tubularia, eudendrium, bimeria, etc.). A good idea of the nature of the compound hydrozoa may be formed from the consideration of the carnpanularia oma, a common organism on our shores. The compound polyp-animal, or association of polyps, resembles a miniature tree. It consists essentially of a ramified tube of irritable matter, defended by an external flexible, and frequently jointed horny skeleton; and is fed by the activity of the tentacula, and by the digestive powers of the alimentary sacs of a hundred polypi, the common produce of which circulates through the tubular cavities for the benefit of the whole community. The soft integument of the nutrient polyps contains the thread-cells, to which allusion has previously been made. These are protruded when the skin is irritated, and give the tentacles the appearance of being beset by minute bristles. The digestive sac of each polyp is lined by a ciliated epithe lium; but there is a perforation at the base communicating with the central tube. This outlet admits only of the passage of the fluid contents of the stomach, undigested mat ters being ejected by the mouth. There is reason to believe that sea-water enters the branches of the tube and circulates, by means of the ciliated epithelium, through the compound organism; and by this means contributes to the respiratory process. " At cer tain points of these ramified polyps," says prof. Owen, "which points are constant in and characteristic of each species, there are developed little elegant vase-shaped or pod shaped sacs, which are called the ovigerous vesicles, or ovicapsules. These are sometimes appended to the branches, sometimes to the axillte. They are at first soft, and have a still softer lining membrane, which is thicker and more condensed at the bottom of the vesicle. It is at this part that the ova or germs are developed, and for some time these are kept in connection with the vital tissue of the polyp by a kind of umbilical cord. In all the compound hydrozoa, the ovicapsules are deciduous; and having performed their functions in relation to the development of the new progeny, drop off like the seed capsules of plants." On other individuals of the same species sperm-capsules are devel oped, which, in form, resemble the ovicapsules, but in place of ova, contain spermatozoa. The act of fertilization in most cases occurs by diffusion of the spermatozoa in the sur rounding water. There is much that still requires elucidation in reference to the vari ous modes of reproduction of this class. Many of the hydrozoa have been shown to be merely larval forms of medusm. See GENERATIONS, ALTERNATION OF The leading anatomical distinction between the anthozoa, or actinozoa, and the hydrozoa has been already noticed. The common actinia (q.v.) may be regarded as the type of this class, all of which are marine, and principally inhabit the warmer or tropi cal seas. Many of the larger tropical polyps of this class combine with a structure simi lar to that of the actinia an internal calcareons axis or skeleton, which, penetrating into the interior of the organism, presents the lamellated and radiated structure recognizable in fungtie, and in the skeletons of caryop1y7,ar. madreporce, etc. Such anthozoa are termed coralligenous: and every hard structure deposited in or by the tissues of this class, and forming a uniform framework, is recognized by zoologists as a coral. Like the members of the preceding class many of the anthozoa multiply freely by gemination, complex or compound animals or colonies of animals being formed, in winch individual polyps are united by a ccenosare or polypidom. For adescription of the mode in which communication takes place between the common body mass and the individual polyps, we must refer to the article ALcvomum. Various arrangements of this class have been proposed by zoologists. If we exclude the consideration of fossil genera, we may divide the authozoa into two orders—the alcyonaria and the zoantharia.