||Met`a*zo"a (?), n. pl. [NL., fr. Gr. &?; after + zo^,on an animal.] (Zoöl.) Those animals in which the protoplasmic mass, constituting the egg, is converted into a multitude of cells, which are metamorphosed into the tissues of the body. A central cavity is commonly developed, and the cells around it are at first arranged in two layers, -- the ectoderm and endoderm. The group comprises nearly all animals except the Protozoa.