1911 Encyclopædia Britannica/Actinozoa

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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.)