Hydrozoa

Hydrozoa, one of the two classes of the Phylum Coelenterata (q.v.). It is separated from the Anthozoa (q.v.) by the fact that the digestive cavity is not separate from the general body cavity, into which the mouth at once opens. The generative products, moreover, are usually discharged to the exterior and not into the body cavity and thus out by the mouth or special pore. Very commonly they are dimorphic, or possess two distinct forms; thus the common hydroid colony Millepora has two quite dissimilar sets of zooids; this is still better shown in the Siphonophora (q.v.), in which the individual polypites are modified to serve very different functions; thus some are reproductive, others nutritive, others protective, and others locomotory. Alternation of generations (q.v.) is also common; a fixed colony often gives rise to free-swimming jellyfish in which the reproductive organs are developed. The gastric cavity or stomach is simple in structure, but a series of four ridges may run along it, and thus increase the digestive surface. The Hydra is the simplest type of the Hydrozoa, but this is probably the result of degeneration rather than of having retained the primitive characters of the order. The Hydrozoa mostly have a soft chitinous skeleton, as e.g. in Sertnlaria (the Sea-fir), but it may be massive and calcareous as in the Millepora. The group is divided into two subclasses: - I. Ceaspedota. Order 1. Trachymedusa; (q.v.).

'2. Hydroida (q.v.).

3. Siphonophora (q.v.).

II. Ackaspeda. 1. Tetrameralia.

2. Octomeratia.