Polyzoa (Gr. , many, and , animal), a name given by Thompson to the lowest of the molluscoids, popularly known as sea mosses and sea mats; Ehrenberg called them bryozoa. They form colonies of distinct similar zooids, protected usually by a horny or chitinous integument. They look much like hydroids, but the separate cells of the colony are merely connected externally, without direct communication with each other. The separate zooids are called polypides, and each is enclosed in a double sac, the outer wall of which, or ectocyst, is chitinous or calcareous, the inner being a delicate endocyst. The mouth is surrounded by ciliated tentacles, the movements of which create currents in the water which bring them their food; these can be more or less retracted into the sac. There is a well marked gullet, stomach, and distinct alimentary canal, with the vent at the upper part of the sac; the nervous system is essentially a single ganglionic mass, between the gullet and the anus, which gives off filaments in various directions; the circulation is carried on by means of cilia, there being no distinct heart, and no definite course to the circulating fluid; respiration is effected chiefly by the crown of ciliated tentacles around the mouth.
There are distinct reproductive organs within the sac; they are all hermaphrodite, the eggs being dropped into the body cavity, where they are fertilized; they also reproduce by continuous budding and discontinuous gemmation. An organ, called the "bird's head process," with pincer-like beaks which are constantly snapping together, has not been assigned for any definite use. Most of the polyzoa are fixed and plant-like; but the fresh-water cristatella colony creeps about on a base flattened like the foot of a slug. In the fresh-water forms the crown of tentacles generally assumes the horseshoe shape, while in the marine it is circular. For descriptions and figures of the fresh-water polyzoa see " American Naturalist," vol. i., 1868. (See Bryozoa).