ACTINOZOA  , a

term in systematic zoology, first used by H . M . D. de Blainville about 1834, to designate animals the organs of which were disposed radially about a centre . De Blainville included in his group many unicellular forms such as Noctiluca (see PROTOZOA), sea-anemones, corals, jelly-fish and hydroid polyps, echinoderms, polyzoa and rotifera . T . H . Huxley afterwards restricted the term . He showed that in de Blainville's group there were associated with a number of heterogeneous forms a group of animals characterized"by being composed of two layers of cells comparable with the first two layers in the development of vertebrate animals . Such forms he distinguished as Coelentera, and showed that they had no special affinity with echinoderms, polyzoa, &c . He divided the Coelentera into a group Hydrozoa, in which the sexually produced embryos were usually set free from the surface of the body, and a group Actinozoa, in which the embryos are detached from the interior of the body and escape generally by the oral aperture . Huxley's Actinozoa comprised the sea-anemones, corals and sea-pens, on the one hand, and the Ctenophora on the other . Later investigations, whilst confirming the general validity of Huxley's conclusions, have slightly altered the limits and definitions of his groups .

(See

ANTHOZOA, COELENTERA, CTENOPHORA and HYDROZOA.) (P . C .