PRO-toe-stomes One of two coelomate lineages, characterized by spiral cleavage, a blastopore that develops into a mouth, and determinate cleavage. 512

Animals in which the Þrst opening that appears in the embryo becomes the mouth; e.g., mollusks, annelids, and arthropods.

Gr. protos - first, primary; Gr. stomatos - mouth]. A group of Metazoans that exhibit determinate, spiral cleavage and that develop a mouth from the blastopore. (ex. Nematodes, Flatworms, Molluscs).