CLASS A. III-DnozoN (Gr. hydra and zo5n, animal; hydra animal). Cmlenterata in which the walls of the digestive cavity entirely coincide with those of the body cavity. The reproductive organs are external processes of the body. They are all aquatic, uhYstly marine. The hydrozoa are divided into six sub-classes: I. EvnuornA. Sec HYDROIDS. This sub-class comprises six orders: Order I. Hydrides. Fresh-water polyp; only one genus, hydra, including various species.
Order II. Corynida (Gr. kOPU lie, a club). Pipe coralline (q.v.), (tubularia). The order is entirely marine. with one exception. The reproductive elements are developed in distinct buds or sacs, which are external processes of the body, called by professor All man, gonosphores. There are great variations in the form of these generative buds. In some species they are mere sae-protuberances called sporostics. There is an advance in structure in the different genera, the gonosphore being sometimes composed of a bell shaped disk, called the gonocalyx.
Order III. Sertala•ida (Lat. septum, a The ttnimals of this order resemble the corynida in becoming permanently fixed after their embryonic condition. Each polypite consists of a soft, contractile and extensible body, having at its distal extremity a mouth surrounded by prehensile tenacles. The internal arrangement of the whole
organism iS exceedingly interesting. See SERTULARIA, PLUMULARIA, and POLYPI.
OrderIV. Campanalarida (Lat. eamp«nuta, a bell). This order resembles sertula rida, but the gonosphores are usually detached as free-swimming medusa-, instead of being pcsrmanently attached.
Order V. Thecomedasa Prof. Allman has recently described a remarkable hydrozoOn winch he regards as the type of a new order. It is always found embedded in a species of sponge, which it permeates by chitinous (see CHITIN) tubes, opening on the surface. See STEPIIANOSCYPIIUS Muumuus.
Order VI. Medusida3 or Hydromedusida (acalepltm in part). • The animals included in this order have often been placed in a distinct sub-class (diseephora) of hydrozoa, but they are now generally regarded as simply an order of hydroid zoi)phytes. They com prise most of the smaller organisms known as jelly-fishes or sea-nettles. from the property of causing a stinging to any part of the human body which may be touched See ACALEPII2E.