Protostomes encompass the world’s simplest animals such as mollusks, flatworms, nematods, including the earliest bilateral animals to appear in the fossil. Protostomia (from Greek meaning “mouth first”) are a clade of animals. A clade is a group consisting of an organism and all its descendants. Together with their counterpart, the Deuterostomes and a few smaller phyla (taxonomic term division), they make up the Bilateria, mostly encompassing animals with bilateral symmetry and three germ layers. The major differences between deuterostomes and protosomes are found in embryonic development. Protostomes and deuterostomes are thought to have split at least 558 million years ago and possibly much earlier. This is inferred by the existence of Kimberella 555 million years ago, a primitive mollusk which is a known protostome. However, just because kimberella existed at that time, deuterostomes did not evolve later, branching off from protostome. There is little solid agreement on this subject, but apparently some Late Ediacaran (older than 542 million years) fossils, Emettia, have likely deuterostome affinity, though agreement on the classification of Ediacaran fossils is frequently absent. In any case, it is certain that protostomes and deuterostomes had separated by the dawn of the Cambrian, 542 million years ago.
Current molecular data suggests that protostome animals can be divides into 3 groups. They are as follows:
Deuterostomes, came from the Greek word, “second mouth’. They are the superphylum of animals. They are a subtaxon of the Bilateria branch and are opposed to the protostomes. Protostomes and Deuterostomes are distinguished by their embryonic development. In deuterostomes, the first opening, the blastopore becomes the anus, while in protostomes, the first opening becomes the mouth. Deuterosomes are also known as “Enterocoelomates” because their coelom, a fluid-filled cavity formed within the mesoderm develops through Enterocoely, which is a process by which embryos, develop.
The most fundamental difference mentioned between both groups is that the mouth is formed in the embryo first, rather than the anus. 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 certainity in where differentiated cells ultimately end up. The protostomes embryologic development might therefore be considered relatively conventional and predictable. In protostomes, the body cavity is formed by schizocoely but in deuterostomes, the body cavity is formed by enterocoely.
When the species count is noticed, protostomes win over deuterostomes. While there are many millions of protostome species, there are only about 70,000 species of deuterostomes, though these include all fairly complex animals. Protosotomes encompass all simple animals, but their abundance is a testimony to their success. If fishes and whales are ignored, protostomes make up the majority of biomass in the world’s oceans, though they are less dominant on land, where the vertebrate body plan has proven highly successful.