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Encyclopedia Britannica - Main :: PIG-POL |
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POLYP , the name given by zoologists to the form of animal especially characteristic of the subphylum 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.). 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
a sac, the wall
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 struc- tureless gelatinous substance termed mesogloea, secreted by the cell-layers of the body
wall
may be a very thin layer, or may reach a fair
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 water polyp. The animal is attached tentacles. Each tentacle is to the stem of a plant, and is repre- a glove- finger
uppermost; the mouth, not actually ing of the whole wall of the seen in the drawing, is at the lower sac and contains typically extremity of the body, surrounded a prolongation of its 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 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
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
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
great
For figures of polyps see P. Gosse, A History of the British Sea-Anemones and Corals (London, 186o) ; A. Andres, " Le Attinie," in Fauna and Flora des Golfes von Neapel, ix. 1 ( Leipzig
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