One of the mysteries of the English language finally explained.
Originally: (A. S. Hyatt's name for) a member of a grade of primitive multicellular animals characterized by a simple central cavity with branching sacs (not now considered coelomic), comprising most or all of the sponges. Now: a metazoan animal considered to represent or resemble a common ancestor of coelomate phyla.
Late 19th century. From proto- + coelomate. Compare scientific Latin protocoelomata.
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