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marine or freshwater animals that form colonies of zooids

marine or freshwater animals that form colonies of zooids

A class of animals forming the lowest members of the Mollusca, and generally known by the popular names of "sea-mosses" and "sea-mats." They are invariably compound, forming associated growths or colonies produced by gemmation from a single primordial individual, and inhabit a polyzoarium, corresponding to the polypidom of the composite hydroids. The typical polypide of a polyzoon differs from the polypite of the Hydrozoa in having a distinct alimentary canal suspended freely in a body cavity, and in having the reproductive organs contained within the body. The body is inclosed in a doublewalled sac, the outer layer (ectocyst) of which is chitinous or calcareous, and the inner (endocyst) a delicate membranous layer. All the Polyzoa are hermaphrodite. Besides true sexual reproduction, and besides the power of producing colonies by continous budding, fresh individuals are in many cases produced by a process of discontinuous gemmation. The Polyzoa are chiefly marine, encrusting stones, old shells, and sea-weeds; but some are fresh-water.