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’. 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). Two main clades of protostomes are now recognized: Ecdysozoa and Lophotrochozoa (some authors propose a third, the Platyzoa). Compare deuterostome.