From ZooTerms (Dictionary of Invertebrate Zoology)
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protostome also protostom (noun; Greek protos, first; stoma, mouth): 1. Metazoans with determinate spiral clevage and where the mesoderm can be traced to a single cell in the blastula, the mouth originates from the blastopore; includes mollusks, annelids, arthropods, nematodes and certain lesser phyla

See also: deuterostome. 2. (NEMATA) In rhabditid-like nematodes, the cylindrical midportion of the stoma, delimited anteriorly by the cheilostome and posteriorly by the telostome; the protostom may be further subdivided into: prostome, mesostome and metastome