clade of animals
Protostomes are a nephrozoan taxon of animals above the level of phylum. It is not part of the normal system of classification.
Temporal range: Ediacaran – Recent
|The caribbean reef squid, an example of a protostome|
They are a subtaxon of the Bilateria, and are contrasted with the deuterostomes. Protosomes are distingued by their embryo: in deuterostomes, the first opening (the blastopore) becomes the anus, while in protostomes it becomes the mouth.
DNA sequence analysis suggests that protostome animals can be divided into three major groups:
- Ecdysozoa, e.g. arthropods, nematodes
- Platyzoa, e.g. platyhelminthes, rotifers
- Lophotrochozoa, e.g. molluscs, annelids
as well as a number of minor taxa, like the Chaetognatha.