Protostome
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| Protostomes Fossil range: Ediacaran - Recent |
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 Caribbean Reef Squid, an example of Protostome
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Protostomia (from the Greek: mouth first) are a taxon of animals. Together with the deuterostomes and a few smaller phyla, they 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. In protostome development, the first opening in development, the blastopore, becomes the animal's mouth. In deuterostome development, the blastopore becomes the animal's anus. Within the Protostomes a number of phyla undergo what is known as spiral cleavage which is determinate, meaning that the fate of the cells is determined as they are formed. This is in contrast to deuterostomes which have radial cleavage that is indeterminate.
Protostomes are schizocoelomates, meaning a solid mass of the embryonic mesoderm splits to form a coelom. Deuterostomes are enterocoelous, meaning the folds of the archenteron form the coelom.
Current molecular data suggest that protostome animals can be divided into three major groups:
- Ecdysozoa, e.g. arthropods, priapulids
- Platyzoa, e.g. platyhelminthes, rotifers
- Lophotrochozoa, e.g. molluscs, annelids
as well as a number of minor taxa of basal or ambiguous affinity.
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