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[proh-tuh-stohm] /ˈproʊ təˌstoʊm/
noun, Zoology
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.
Compare deuterostome.
Origin Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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protostome in Science
Any of a major group of animals defined by its embryonic development, in which the first opening in the embryo becomes the mouth. At this stage of development, the later specialization of any given embryonic cell has already been determined. Protostomes are one of the two groups of animals having a true body cavity (coelom) and are believed to share a common ancestor. They include the mollusks, annelids, and arthropods. Compare deuterostome.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Article for protostome

group of animals-including the arthropods (e.g., insects, crabs), mollusks (clams, snails), and annelid worms-classified together on the basis of embryological development. The mouth of the Protostomia (proto, "first"; stoma, "mouth") develops from the first opening into the embryonic gut (blastopore). The coelom (body cavity) forms from a split in the embryonic mesoderm (middle tissue). Larval (immature) forms, if present, are called trochophores. The Protostomia constitute one of two divisions of the coelomates (animals with a body cavity, or coelom). Compare Deuterostomia.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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