Protostoma
| Protostomes Temporal range: Ediacaran - Recent |
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|---|---|
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| A centipede (Ecdysozoa) | |
| Scientific classification | |
| Kingdom: | Animalia |
| Subkingdom: | Eumetazoa |
| Clade: | Bilateria |
| Clade: | Nephrozoa |
| (unranked): |
Protostomia Grobben, 1908 |
| Superphyla | |
Protostomia (from Greek πρωτο- proto- "first" and στόμα stoma "mouth") is a clade of animals. 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: most (but, confusingly, not all) protostomes undergo protostomy, whereas most deuterostomes undergo deuterostomy.
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 was once believed that in all cases the embryological dent formed the mouth while the anus was formed later, at the opening made by the other end of the gut. It is now known that the fate of the blastopore in protostomes is extremely variable. While the evolutionary distinction between deuterostomes and protostomes remains valid, the descriptive accuracy of the name 'protostome' (in Greek: "first-mouth") is disputable.
Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial cleavage during cell division, while many protostomes (the Spiralia) undergo spiral cleavage. Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the embryonic gut develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily. Most protostomes have schizocoelous development, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by enterocoelic pouching, through invagination of the endoderm.
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