/ Dictionary / Index P protostome: Meaning and Definition of Find definitions for: pro•to•stome Pronunciation: (prō'tu-stōm"), [key] — n. Zool. 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. protostele prototherian Related Content Daily Word Quiz: feline Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language