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Meaning of protostome

pro•to•stome

Pronunciation: (prō'tu-stōm"), [key]
— n. Zool.
  1. 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.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.