protostom

n. [Gr. 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 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.