| Protostomes Temporal range: Ediacaran–Recent |
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|---|---|
| A Caribbean Reef Squid, an example of a protostome. | |
| Scientific classification | |
| Kingdom: | Animalia |
| Subkingdom: | Eumetazoa |
| (unranked): | Protostomia Grobben, 1908 |
| Superphylums | |
Protostomia (from Greek meaning "mouth first") are a clade 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 animals at least as complex as earthworms, the embryo forms a dent on one side, the blastopore, which deepens to become the archenteron, the first phase in the growth of the gut. In deuterostomes, the original dent becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. The protostomes were so named because it used to be thought that in their embryos the dent formed the mouth while the anus was formed later, at the opening made by the other end of the gut. More recent research, however, shows that in protostomes the edges of the dent close up in the middle, leaving openings at the ends which become the mouth and anus.[1] However, this idea has been challenged, because the Acoelomorpha, a group which forms a sister group to the rest of the bilaterian animals, have a single mouth which leads into a blind gut (with no anus). The genes employed in the embryonic construction of this mouth are the same as those expressed around the protostome mouth, [2]
There are other significant differences between the protostome and deuterostome patterns of development:
Current molecular data suggest that protostome animals can be divided into three major groups:
as well as a number of minor taxa of basal or ambiguous affinity.
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