Metazoa \Met`a*zo"a\, n. pl. [NL., fr. Gr. ? after + ? an animal.]

(Zo["o]l.) 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.

Source: Webster's Revised Unabridged Dictionary (1913)