An animal in which the mouth develops from the opening (blastopore) of the embryonic cavity (see archenteron). The name derives from Greek, meaning literally ‘first mouth’. Protostomes constitute one of the two major subkingdoms of animals (Protostomia), the other being the Deuterostomia (see deuterostome). Other features typical of protostomes are spiral cleavage of the blastula, determinate development (i.e. the fate of cells is established at a very early embryonic stage), and schizocoelic formation of a coelom where one occurs (i.e. by formation of a cavity within a solid mass of mesoderm). Three main clades of protostomes are now recognized: Ecdysozoa; Lophotrochozoa; and Platyzoa.
Subjects: Biological Sciences.