Platynereis dumerlii
Acropora millepora
It all started several years ago. Scientist were trying to decide the relationships between vertebrates, arthropods and nematodes. There were two competing theories. The first is based on the fact that the nemetodes lack a true body cavity, rather they have a false coelom sandwiched between endoderm and mesoderm tissue layers.
Coelemates, on the other hand, have a true body cavity.
The Coelemates are divided into deuterostomes (which includes vertbrates) and protostomes (based on whether the blastophore develops into the anus or the mouth respectively).
The second, based on small subunit ribosomal RNA analysis, revamps this placing acoelemates (lacking a coelom) and pseudocoelomates (such as nematodes) in a clade with protostomes. More specifically, they placed nematodes in a clade with arthropods. This clade is defined based on the fact that they moult. Both phylogenies are pictured below.
This is where things get really interesting. There have been a number of studies designed to settle the question. During the course of the research some interesting, and unexpected phenomena came to light. In the meantime, researchers studying primates discovered theat the rate of molecular evolution in humans and apes was slower than the rate of molecular evolution in monkeys….
Look for Part Two tomorrow (hopefully).
Afarensis: Anthropology, Evolution, and Science 
Both Arthopoda and Annelida have segments, so are they saying segmentation is an independantly derived character? Also this “new view” would mean the coelom also developed twice.
Are they throwing out morphology data to justify molecular data, or is there more to back it up than molting?
But I admit I don’t undstand the importance of the coelom. Why is having a lined cavity (coelemates) such a big deal? Platyhelminthes have a gastrvascular cavity, but its not lined in mesoderm so they are “acoelomate” I don’t understand why it’s not a “real” cavity.
As I point out in a later post, this is really far afield for me, although I found the whole thing interesting (mainly, these posts were about starting with a paper I found interesting and following references to see where I would end up). Most of the stuff I read was about the molecular data. As I understand it, the differences between the three (coelemate, acoelemate and pseudocoelemates)is embryological. One of the implications is that the acoelemates and psuedocolemates would more derived. I am not aware of other morphological traits, besides the moulting, that link them but you might check out Giribet’s 2003 article in Zoology (106:303-326) or Aguinaldo et al’s article in Nature (1997, 387:489-93, which seems to be the paper that started the Ecdysozoa hypothesis). You might also check out the PZ Myers post I linked to in a later post.
As I point out in a later post, this is really far afield for me, although I found the whole thing interesting (mainly, these posts were about starting with a paper I found interesting and following references to see where I would end up). Most of the stuff I read was about the molecular data. As I understand it, the differences between the three (coelemate, acoelemate and pseudocoelemates)is embryological. One of the implications is that the acoelemates and psuedocolemates would more derived. I am not aware of other morphological traits, besides the moulting, that link them but you might check out Giribet’s 2003 article in Zoology (106:303-326) or Aguinaldo et al’s article in Nature (1997, 387:489-93, which seems to be the paper that started the Ecdysozoa hypothesis). You might also check out the PZ Myers post I linked to in a later post.