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Definition of PROTOSTOME
: any of a major group (Protostomia) of bilateral metazoan animals (as mollusks, annelids, and arthropods) characterized in typical forms by determinate and spiral cleavage, formation of a mouth and anus directly from the blastopore, and formation of the coelom by splitting of the embryonic mesoderm — compare deuterostome
Origin of PROTOSTOME
New Latin Protostomia, from prot- + Greek stoma mouth — more at stomach