1911 Encyclopædia Britannica/Polyp
POLYP, the name given by zoologists to the form of animal especially characteristic of the sub phylum Cnidaria of the Coelentera (q.v.). In the subdivision Anthozoa, comprising the sea-anemones and corals, the individual is always a polyp; in the Hydrozoa, however, the individual may be either a polyp or a medusa (q.v.).
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| Fig. 1.—Hydra viridis, the fresh-water polyp. The animal is attached to the stem of a plant, and is represented with the base of attachment uppermost; the mouth, not actually seen in the drawing, is at the lower extremity of the body, surrounded by the circle of tentacles. ov, Ovary; te, testis. |
A good example of a polyp may be seen in a common sea-anemone or in the well-known fresh-water polyp, Hydra (fig. 1). The body may be roughly compared in structure to a sac, the wall of which is composed of two layers of cells. The outer layer is known technically as the ectoderm, the inner layer as the endoderm. Between ectoderm and endoderm is a supporting layer of structureless gelatinous substance termed mesogloea, secreted by the cell-layers of the body-wall; the mesogloea may be a very thin layer, or may reach a 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 firm object by its blind end, and bears at the upper end the mouth surrounded by a circle of tentacles. Each tentacle is a glove-finger-like outpushing of the whole wall of the sac and contains typically a prolongation of its internal cavity, so that primarily the tentacles are hollow; but in some cases the tentacle may become solid by obliteration of its cavity. The tentacles are organs which serve both for the tactile sense and for the capture of food. By means of the stinging nettle-cells or nematocysts with which the tentacles are thickly covered, living organisms of various kinds are firmly held and at the same time paralysed or killed, and by means of longitudinal 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-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 column, circular or oval in section, forming the trunk, resting on a base or foot and surmounted by the crown of tentacles, which enclose an area termed the peristome, in the centre of which again is the mouth. As a 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 simple structure.
The name polyp was given to these organisms from their supposed resemblance to an octopus (Fr. poulpe), with its circle of writhing arms round the mouth. This comparison, though far-fetched, is certainly more reasonable than the common name “coral-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 insect as it is from man himself.
The external form of the polyp varies greatly in different cases. In the first place the column may be long and slender, or may be, on the contrary, so short in the 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 mere knobs or warts; they may be simple and unbranched, or they may be feathery in pattern. All these types are well illustrated by different species of British sea anemones. The mouth may be level with the surface of the peristome, or may be projecting and 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 power of “reproducing themselves non-sexually by the method of budding. This mode of reproduction may be combined with sexual reproductiveness, or may be the sole method by which the polyp produces offspring, in which case the polyp is entirely without sexual organs. In many cases the buds formed do not separate from the parent but remain in continuity with it, thus forming colonies or stocks, which may reach a great size and contain a vast number of individuals. Slight differences in the method of budding produce great variations in the form of the colonies, which may be distinguished in a general way as spreading, massive or arborescent. The reef-building corals are polyp-colonies, strengthened by the formation of a firm skeleton. For further details of colony formation the reader is referred to the articles Anthozoa and Hydromedusae.
For figures of polyps see P. Gosse, A History of the British Sea-Anemones and Corals (London, 1860); A. Andres, “Le Attinie,” in Fauna and Flora des Golfes von Neapel, ix. 1 (Leipzig, 1884); G. J. Allman, A Monograph of the Gymnoblastic or Tubularian Hydroids (Ray Society, 1871–1872). (E. A. M.)