POLYP , the name given by zoologists to the
See also:form of 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 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, 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: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 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 capture of
See also:food .
By means of the stingingnettle-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: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 section, forming the 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 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: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 man himself . The
See also: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
See also: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
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 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 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: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
See also:building corals are polyp-colonies, strengthened by the formation of a firm
See also:skeleton . For further details of colony formation the reader is referred to the articles ANTHOZOA and
See also:HYDROMEDUSAE . For figures of polyps see P . 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:des Golfes von Neapel, ix .
See also:Leipzig, 1884) ; G . J .
See also:Allman, A Monograph of the Gymnoblastic or Tubularian Hydroids (Ray Society, 1871-1872) . (E . A .
MICRONESIA MELANESIA POLYNESIA (q.v.)
POLYPERCHON (incorrectly Polysperchon)
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