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POLYP

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Originally appearing in Volume V22, Page 37 of the 1911 Encyclopedia Britannica.
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POLYP , the name given by zoologists to the See also:

form of See also:animal especially characteristic of the subphylum Cnidaria of the See also:Coelentera (q.v.). In the subdivision See also:Anthozoa, comprising the See also:sea-anemones and See also:corals, the individual is always a polyp; in the See also:Hydrozoa, however, the individual may be either a polyp or a See also:medusa (q.v.). A See also:good example of a polyp may be seen in a See also:common sea-See also:anemone or in the well-known fresh-See also:water polyp, See also:Hydra (fig. 1). The See also:body may be roughly compared in structure to a See also:sac, the See also:wall of which is composed of two layers of cells. The See also:outer layer is known technically as the ectoderm, the inner layer as the endoderm. Between ectoderm and endoderm is a supporting layer of struc- tureless gelatinous substance termed mesogloea, secreted by the See also:cell-layers of the body-wall; the mesogloea may be a very thin layer, or may reach a See also:fair thickness, and then sometimes contains skeletal elements formed by cells which have migrated into it from the ectoderm. The sac-like body built up in this way is attached usually to some See also:firm See also:object by its See also:blind end, and bears at the upper end. the mouth water polyp. The animal is attached tentacles. Each tentacle is to the See also:stem of a plant, and is repre- a See also:glove-See also:finger-like outpushsented with the See also:base of See also:attachment uppermost; the mouth, not actually See also:ing of the whole wall of the seen in the See also:drawing, is at the See also:lower sac and contains typically extremity of the body, surrounded a prolongation of its See also:internal by the circle of tentacles. ov, Ovary ; cavity, so that primarily the te, testis. tentacles are hollow; but in some cases the tentacle may become solid by obliteration of its cavity. The tentacles are See also:organs which serve both for the tactile sense and for the See also:capture of See also:food.

By means of the stinging See also:

nettle-cells or nematocysts with which the tentacles are thickly covered, living organisms of various kinds are firmly held and at the same See also:time paralysed or killed, and by means of See also:longitudinal See also:muscular fibrils formed from the cells of the ectoderm the tentacles are contracted and convey the food to the mouth. By means of circularly disposed muscular fibrils formed from the endoderm the tentacles can be protracted or thrust out after contraction. By muscle-See also:fibres belonging to the same two systems the whole body may be retracted or protruded. We can distinguish therefore in the body of a polyp the See also:column, circular or See also:oval in See also:section, forming the See also:trunk, resting on a base or See also:foot and surmounted by the See also:crown of tentacles, which enclose an See also:area termed the peristome, in the centre of which again is the mouth. As a See also:rule there is no other opening to the body except the mouth, but in some cases excretory pores are known to occur in the foot, and pores may occur at the tips of the tentacles. Thus it is seen that a polyp is an animal of very See also:simple structure. The name polyp was given to these organisms from their supposed resemblance to an See also:octopus (Fr. poulpe), with its circle of writhing arms See also:round the mouth. This comparison, though far-fetched, is certainly more reasonable than the common name " See also:coral-See also:insects " applied to the polyps which form coral. It cannot be too emphatically stated that a coral-polyp is as far removed in organization from either an octopus or an See also:insect as it is from See also:man himself. The See also:external form of the polyp varies greatly in different cases. In the first See also:place the column may be See also:long and slender, or may be, on the contrary, so See also:short in the See also:vertical direction that the body becomes disk-like. The tentacles may number many hundreds or may be very few, in rare cases only one or two, or even absent altogether; they may be long and filamentous, or short and reduced to See also:mere knobs or warts; they may be simple and unbranched, or they may be feathery in See also:pattern.

All these types are well illustrated by different See also:

species of See also:British sea-anemones. The mouth may be level with the See also:surface of the peristome, or may be projecting and See also:trumpet-shaped. As regards internal structure, polyps exhibit two well-marked types of organization, each characteristic of one of the two classes, Hydrozoa and Anthozoa. It is an almost universal attribute of polyps to possess the See also:power of reproducing themselves non-sexually by the method of budding. This mode of See also:reproduction may be combined with sexual reproductiveness, or may be the See also:sole method ,by which the polyp produces offspring, in which See also:case the polyp is entirely, without sexual organs. In many cases the buds formed do .not See also:separate from the See also:parent but remain in continuity with it, thus forming colonies or See also:stocks, which may reach a See also:great See also:size and contain a vast number of individuals. Slight See also:differences in the method of budding produce great See also:variations in the form of the colonies, which may be distinguished in a See also:general way as spreading, massive or arborescent. The See also:reef-See also:building corals are polyp-colonies, strengthened by the formation of a firm See also:skeleton. For further details of See also:colony formation the reader is referred to the articles ANTHOZOA and See also:HYDROMEDUSAE. For figures of polyps see P. See also:Gosse, A See also:History of the British Sea-Anemones and Corals (See also:London, 186o) ; A. See also:Andres, " Le Attinie," in See also:Fauna and See also:Flora See also:des Golfes von Neapel, ix.

1 (See also:

Leipzig, 1884) ; G. J. See also:Allman, A Monograph of the Gymnoblastic or Tubularian Hydroids (See also:Ray Society, 1871-1872). (E. A.

End of Article: POLYP

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