A multicellular organism whose mouth develops from a primary embryonic opening, such as an annelid, mollusc, or arthropod.
- These animals are all protostomes - the mouth develops before the anus in the young embryo - and they have long been recognized as belonging together as a group.
- In the complementary category, the protostomes, which includes annelids and arthropods, the blastopore develops into the mouth.
- Arthropods are bilaterally symmetrical protostomes with strongly segmented bodies.
1950s: from proto- 'primitive' + Greek stoma 'mouth'.
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