Hydrozoa

(hi-dro-20’a), n. pl. [NL., pl. of hy- rozoon, q. vy.) A cla of Cal enter at a, one of two prime divisions of that subkingdom, Actinozoa being the other. The Hydrozoa are aquatic and chiefly marine organisms, single or oftener compound, and nearly always soft and gelatinous (in some cases with a chitinous perisarc), Each individual polypite consists Diagrams of Principal Forms of the Hydrozoa in their mutual relations. 1, hydra; 2, sertularian; 3, calycophoran; 4, physophoridan; 5, lucernarian. 1, II, IV, succe ive stages of a medusiform gonophore, or medusoid. In any figure: a, ectoderm; 4, endoderm; ¢, digestive and somatic cavity; A, air-vesicle; A, hydrophyllium; C, hydrotheca; F, pneumatophore; G, gonophore; JV, nectocalyx; #, tentaculum; S, hydranth; 7, ccenosarc. e entially of a simple sac composed of an outer (ectodermal) and an inner (endodermal) membrane, with a simple gastrovascular cavity or stomach-sac, not differentiated into an esophageal tube nor separated from the general body-cavity, developed as an outward proce of the body wall, and usually furnished with tentacular proce es. The reproductive organs are external to the body. Reproduction is accomplished either by sexual elements (ova and spermatozoa) or by gemmation or fi ion; the generative zoids are developed as medusoid organisms, which may become detached and free-swimming, or remain permanently attached to the parent stock. The cla is of world-wide distribution, and includes the numerous creatures known as hydroids, acalephs, medusans, jelly-jish, sea blubbers, etc. Their forms are endle ly varied, and range in complexity from the simple fresh-water hydra to the complicated structure of the oceanic hydroids, as the Portuguese man-of-war. The cla ification of the Hydrozoa varies with different writers, and it is difficult to define most of the larger groups into which they have been diyided. They are separated into from three to six groups, as the Hydrophora, Discophora, and Siphonophora of Huxley’s arrangement, or the Hydroid a, Siphonophora, Lucern arid a, Discophora, Graptolitide, and Hydrocoralline of Nicholson’s arrangement. The cla is sometimes called Polypomeduse, It was named as a cla of Polypi by Owen in 1843.: