Radiata, Or Radiates, next to the protozoa the lowest of the great branches of the invertebrates, whose characteristic feature is that of radiation from the mouth as a centre. All live in the water, and most are marine. They were divided by Agassiz into polyps, acalephs or jelly fishes, and echinoderms, the last class the highest, which have been described under these titles respectively. As they are among the lowest in rank in the animal kingdom, they are among the earliest in time. Huxley divides the old branch of radiates into the subkingdom coelenterata, including the hydroids, sea anemones, corals, and acalephs; and (in part) the subkingdom an-nuloida, including the echinoderms. In the latter subkingdom he places also the intestinal and some minute aquatic worms, an association not generally accepted by naturalists. His classification, in detail, is as follows: Sub-kingdom coelenterata, having the alimentary canal communicating freely with the body cavity; with no heart or circulating system, and in most with no nervous system.
Class A, hydrozoa, with walls of the digestive sac not separated from those of the body cavity, with the reproductive organs external; containing subclasses I., hydroida (hydroid zoophytes), with orders: 1, hydrida (hydra); 2, corynida (tubularia); 3, sertularida (sea firs); II., siphonophora (oceanic), with orders: 4, calycophoridoe (diphyes); 5, physophoridoe (Portuguese man-of-war); III., discophora (jelly fish), with order 6, medusidoe; IV., lu-cernarida (sea blubbers), with orders: 7, lu-cernariadoe; 8, pelagidoe; 9, rhizostomidoe; V., graptolitidoe (extinct). Class B, actinozoa, with stomach opening into body cavity, which is divided into compartments by vertical partitions, and with reproductive organs internal; with orders: 1, zoantharia, with rounded tentacles in multiples of five or six, as the sea anemones, star and brain corals, and madrepores; 2, alcyonaria, with fringed tentacles in multiples of 4, as alcyonium, tubipores, sea pens, and red coral; 3, rugosa (extinct); 4, ctenophora, oceanic jelly fishes like Venus's girdle and pleurobrachia. In the subkingdom annuloida, the alimentary canal is shut off from the body cavity, and there is a distinct nervous system, generally a blood-circulating system, and a water-vascular system.
The only class which concerns the radiates is the echinodermata, with the five living orders of crinoids, ophiurans, star fishes, sea urchins, and holothurians, and the two extinct low orders of blastoids and cystoids, allied to crinoids. - See a series of papers on "The Mode of Growth of the Radiates," by Prof. Packard, in the "American Naturalist," March, 1875, et seq.