The Collaborative International Dictionary of English v.0.48:

Metazoa \Met`a*zo"a\, n. pl. [NL., fr. Gr. ? after + zo^,on an
   animal.] (Zool.)
   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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Metazoa
    n 1: multicellular animals having cells differentiated into
         tissues and organs and usually a digestive cavity and
         nervous system [syn: Metazoa, subkingdom Metazoa]