Protostomia (from Greek meaning `mouth first`) are a clade of animals. Together with the deuterostomes and a few larger phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers. The major distinctions between deuterostomes and protostomes are found in embryonic development. In animals at least as.....
Synonym: blastopore. ... Origin: proto-+ G. Stoma, mouth ... (05 Mar 2000) ... 
Invertebrate phylum in which the mouth forms from the embryonic blastopore. Major protostome phyla are Annelida, Mollusca and Arthropoda. See deuterostome.
Type: Term Pronunciation: prō′tō-stōm Synonyms: blastopore
any member of the lower invertebrate phyla in which the mouth appears before the anus during development, cleavage is spiral and determinate, and the coelom forms as a splitting of the mesoderm. Cf.