Protostome

From Freepedia

Protostomes (from the Greek: first the mouth) are a taxon of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers. The major distinctions between deuterostomes and protostomes are found in embryonic development. In protostomes development, the mouth forms at the site of the blastopore, and the anus forms as a second opening.

It seems the protostomes evolved from the deuterostomes. That would mean that deuterostomy is the ancestral and original condition and the most primitive way for the Bilateria to develope from eggs to adults. At some point an early stemgroup of deuterostomes would have been split into two different branches, one leading to the modern deuterostomes and the other to the protostomes. The branch that was leading to protostome development must have continued to be deuterostomes a while after the split, until some remote ancestor evolved into the first protostome which gave rise to all the other present and extinct species and groups within this highly successful group. It probably only happened once. The only surviving "non-protostomian" group from the ancient deuterostomian lineage that would eventually resulted in the protostomes would have been the ancestors of the Chaetognatha (arrow worms), which still has a deuterostomian development. Evolving a different gastrulation and another way to turning the blastopore into a new individual, which is both backwards and upside down compared to how the blastopore evolved in their ancestors, would have been very difficult for the modern and advanced species of today to do. Because it is likely the transformation happened a very long time ago, when the morphology of the animals was much simplier and with a much more adaptable physiology in the early stages of life than present forms, it was possible to experiment with their morphogenesis in fundamental new ways. And judging from all the protostomians around us, both in numbers and species, it was a huge success.

Current molecular data suggest that protostome animals can be divided into three major groups:

• Ecdysozoa, e.g. arthropods and roundworms
• Lophotrochozoa, e.g. mollusks and annelids
• Platyzoa, e.g. flatworms

Of these, the latter two make up the Spiralia, including most animals where the embryo undergoes spiral cleavage.