"Protostomia (from Greek πρωτο- proto- "first" and στόμα stoma "mouth") is a clade of animals containing phyla including the arthropods, annelids, and molluscs. Together with the deuterostomes and xenacoelomorpha, its members make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers. The major distinctions between deuterostomes and protostomes are found in embryonic development and is based on the embryological origins of the mouth and anus.
In most, but not all protostomes, the mouth forms first, then the anus, whereas the reverse is true in deuterostomes." - (en.wikipedia.org 07.12.2019)
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