Polyp (Gr. , many, and , foot), a name formerly applied to the three classes of ra-diata, the eoral animals and actinia, ,jelly fishes or medusae, and the echinoderms. The name as thus extended was given from the numerous prehensile organs around the mouth, like those of the cephalopods (cuttle fishes); now it is generally restricted to the first class, called zoophytes by Prof. J. D. Dana in his " Report" (8vo, New Haven, 1859). He defined polyps as radiated animals, usually attached at the base, with a eoronet of tentacles above and a • toothless mouth in the centre, with an inner alimentary cavity to which the mouth is the only opening; they are hermaphrodite, reproducing by buds and eggs, with very imperfect circulation and no special organs of sense. Prof. Verrill has since divided polyps, excluding the hydroids, into the three orders of alcy-onaria, actinaria, and madreporaria. Alcyo-naria include polyps with eight (occasionally six) long pinnately lobed tentacles around a narrow disk, compound by budding, with the three suborders: 1, pennatulacea or sea pens, forming free-moving communities; 2, gorgona-cea, whose polyps are cylindrical, short, connected laterally, and secreting a solid central axis, varying in form, and often very delicate and beautiful; 3, alcyonacea, comprising polyps turbinate at the base, usually incrusting other bodies; here belongs the organ-pipe coral.
Actinaria, with conical or cylindrical tentacles, and ambulacral spaces always open, have the three suborders actinacea, antipathacea, and zoanthacea, including the free sea anemones and several fixed families. Madreporaria have polyps simple or compound, with expanded form, simple conical tentacles, solid coral being usually deposited in their tissues or partitions; they are mainly confined to warm seas, and include the madrepores, astraeas, fungias, brain corals, and others which go to make up the ordinary coral reef. Prof. Dana, in his " Corals and Coral Islands" (1872), makes the three great subdivisions: 1, actinoids, related to the anemones, with tentacles and septa a multiple of six; 2, cyathophylloids, with tentacles and septa a multiple of four; these were the earliest corals, and most abundant in palseozoic time; 3, alcyonoids, with eight fringed tentacles like gorgonia. He adopts the tribes of Prof. Verrill, except that he unites the coral-making and non-coral-making species into the one division of actinoids, and separates from these the cyathophylloids. (See Actinia, and Coral; and for details of structure, mode of growth, and figures, the works of Profs. Verrill and Dana).