Protostomes (from the Greek: first the mouth) 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 its mouth. In deuterostome development, the blastopore becomes the animal's anus. Protostomes have what is known as spiral cleavage which is determinate, this 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.

Current molecular data suggest that protostome animals can be divided into three major groups:

• Ecdysozoa, e.g. arthropods and roundworms
• Lophotrochozoa, e.g. molluscs and annelids
• Platyzoa, e.g. flatworms

Of these, the latter two make up the Spiralia, including most animals where the embryo undergoes spiral cleavage.

Animals

أوليات الفم | Prvoústí | Urmünder | Protostomia | Prabuŝulo | Protostomia | Protostomia | Protostomata | Protostomier | Protostomia | Pierwouste | Первичноротые | Prvoústovce | 原口动物