Polyzoa, or BRYOZOA, a class of small animals which, with one exception, form colonies, and are almost always fixed. Most familiar are the sea-mats or horn-wracks (Flustra), cast-up pieces of which are abundant on the beach. On these will be seen the hundreds of separate chambers in which the minute individuals live. Each individual has a sac-like or cup-shaped body, traversed by a food-canal bent like a U, crowned around the mouth by a wreath of tentacles, controlled by a single nerve-centre. The cuticle which surrounds the body is usually horn-like, not unfrequently calcareous (Cellepora, Lepralia, &c.), and sometimes gelatinous (Aleyonidium, Lophopus). The individuals of a colony are not always all alike; thus, some of them are occasionally modified into strange birds'-beak-like or whip-like structures. All Polyzoa multiply by budding, and thus the colonies increase. The individuals in the older parts of the colony usually degenerate or die. Fresh-water forms reproduce by peculiar winter-buds or statoblasts, which are liberated on the death of the parent, are floated away by currents, and after a winter's quiescence develop in spring. But all Polyzoa also reproduce sexually; the sexes may be separate or united; the larvæ developed from the eggs are free-swimming. The Polyzoa used to be ranked with zoophytes (among the Hydrozoa), but the individual animals are much more complex and are independent of one another. Often they are called molluscoid, because of apparent affinities with lamp-shells or Brachiopods, which used to be regarded as allied to molluscs. Most modern zoologists rank them as a distinct but heterogeneous class in the great assemblage of 'worms' or 'Vermes.' Representative genera are Cristatella, Lophopus, Plumatella—in fresh water; Flustra, Membranipora, Aleyonidium, Cellepora—marine; Pedicellina and Loxosoma—two marine genera, simpler than the others, the latter non-colonial. Rhabdopleura, a remarkable genus sometimes included in this class, shows at least hints of vertebrate affinities.
See Allman, British Fresh-water Polyzoa (Lond. 1886); Busk, Challenger Report, X. (1884); Hincks, British Marine Polyzoa (Lond. 1880); E. Ray Lankester, article 'Polyzoa' in Ency. Brit.