The true coelomates are often categorized as either protostomes or deuterostomes. As we discussed in the previous tutorial, the distinction is based on differences in their early cell cleavage, coelom formation, and the fate of the blastopore.
One of the main differences between these two groups is in the origin of their gut. The mouth develops first in protostomes, whereas in deuterostomes the anus develops first (the mouth forms secondarily). However, recent studies have shown that while some groups of protostomes all have the mouth developing from the blastopore (the Platyhelminthes, Nematodes and Rotifers), in other groups it is most of the species (the Mollusks and Annelids), while only a few Arthropods show this pattern. Those studies also show that while all deuterostomes (Echinoderms and Chordates) have the anus developing from the blastopore, so do most Arthropods. However, Arthropods show the protostome condition for both cleavage and the development of the mesoderm. So, while some of these developmental distinctions may not be as clear cut as we once thought, they can still be useful for categorizing animal diversity. We'll begin our discussion with the coelomate protostomes in the phylum Mollusca.