Quick follow up on Neurozoans. This is a brief discussion on the neurology of molluscs. It didn't fit with the larger discussion but I thought it was interesting.
There are seven rough groups within molluscs-- I say "rough" because there's a lot of discussion over who belongs with whom. (See here.) These are:
Bivalves don't have a CNS. Neither do monoplacophora. Gastropoda and Scaphopoda have similar central nervous systems in that they have ganglia that interconnect. These are not usually considered a brain as such though some complexity can be achieved. Aplacophora look simpler than snails in that they have a cerebral ganglion but it's not terribly complex. Polyplacophora (chitons) have no true ganglia but there is a ring of neural tissue around the esophagus. The Cephalopoda have highly developed brains.
Are bivalves the primitive state and the other two classes evolved brains on their own? Or did the primitive mollusc have a brain and lost it in bivalves?
Mollusc clades suggest the that there are two great divisions within the mollusc phylum. One side leads to both bivalves and cephalopods. The other leads to chitons and Aplacophora. Neither chitons or aplacophorans have brains but they do have central ganglia. Over on the other side while bivalves don't have any CNS, gastropods, cephalopods and scaphopods do. Since both sides of the same phyla have at least central ganglia, we can presume that the primitive ancestor of molluscs had one, too, and that bivalves lost theirs.
This is supported by a paper in 2001 by Reichert and Simeone. By analyzing small rRNA sequences, they were able to redraw phylogenetic relationships between phyla and push back the origin of the central nervous system to the very first bilaterally symmetric animals. They further speculate that the original ur-bilateral animal might have had significantly complex brains and that subsequent simplifications of the brain (such as in clams) is just the complexity of the brain degenerating.