What are Protostomes?
Protostomes (Greek: mouth first) are a huge clade of animals that includes the Ecdysozoans (arthropods, nematodes, and friends), Platyzoa (flatworms and allies), and Lophotrochozoa (molluscs, annelids, and allies). Their counterpart, the deuterostomes, includes the chordates, hemichordates, and echinoderms. There are many differences between the two groups, which make up all bilateral animals, but the most frequently mentioned is that the mouth is formed in the embryo first, rather than the anus. This is a pretty fundamental difference. Another difference between protostomes and deuterostomes is that in protostomes the embryo displays spiral cleavage, where the position of each cell is deterministic. In deuterostomes, which display radial cleavage, there is less certainty in where differentiated cells ultimately end up. The protostomes’ embryological development might therefore be considered relatively conservative and predictable. Unsurprisingly, protostomes encompass the world’s simp
