Temporal range: Ediacaran - Recent
|A centipede (Ecdysozoa)|
Protostomia (from Greek - proto- "first" and 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 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. Yet two of the most basal and ancient protostome phyla, the chaetognatha and priapulida, have deuterostome-like development.
The common ancestor of protostomes and deuterostomes was evidently a worm-like aquatic animal. The two clades diverged about 600 million years ago. Protostomes evolved into over a million species alive today, compared to about 60,000 deuterostome species.
Protostomes are divided into the Ecdysozoa, e.g. arthropods, nematodes; the Spiralia, e.g. molluscs, annelids, platyhelminths, and rotifers. A modern consensus phylogenetic tree for the protostomes is shown below. The timing of clades radiating into newer clades is given in mya (millions of years ago); less certain placements are indicated with dashed lines.