Metazoa

‖Met′a‐zo″a (?), n. pl. [NL., fr. Gr. � after + ζο̑ͅον 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.