Protostomes (from the Greek: first the mouth) are a superphylum of animals in the taxonomic group bilateria, and include animals such as arthropods, mollusks, and nematodes. They are most often compared with deuterostomes, the other major group of bilateria. The major distinctions between deuterostomes and protostomes are found in embryonic development.
In both protostomes and deuterostomes, the embryo consists of a little ball of cells known as a blastula. Protostomes have their early cell divisions diagonal to the polar axis forming a spiral arrangement of cells; this is called spiral cleavage. A groups of cells move inward to form an opening called the blastophore, which in protostomes develops into the mouth.
Protostomes have a determinate cleavage: the fate of how each embryonic cell will turn out to be or function is typically fixed very early; the first four cells are separate and each will develop into a fixed quarter of the larva. If a cell is removed from the blastula, a limb might not form, for the other cells don't compensate. Protostomes are schizocoely, where the mesoderm splits and the split widens into a cavity that becomes the coelom.
In protostomes development, the mouth forms at the site of the blastopore, and the anus forms as a second opening.
Current molecular data suggest that protostome animals can be divided into two major groups: lophotrochozoa and ecdysozoa.
The protostome condition is defined by a spiral and determinant cleavage in the early stages of embryo development, schizocoelous coelom formation (as the archenteron (embryonic gut) forms the coelom begins as splits within the solid mesodermal mass, and the formation of the blastopore (the original opening) into the mouth
There is a tendency among researchers in the field of molecular phylogeny to divide the Protostomia into two further groups, the Ecdysozoa and Lophotrochozoa.
Paleontologist Conway Morris presents a different tree again, and it is clear that there is a lot to be resolved before we can have a clear picture of the early branchings of the animal evolutionary tree.
The simplest of such animals are the Platyhelminthes (flatworms), which may be paraphyletic to the higher phyla.
The vast majority of the triploblastic phyla form a group called the Protostomia.
These phyla all have a complete digestive tract (including a mouth and an anus), with the mouth developing from the archenteron and the anus arising later.
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