The six thousand species of marine animals in the phylum Echinodermata ("spiny-skinned") are, like annelids, arthropods, chordates, and mollusks, characterized by a true coelom, or body cavity. However, echinoderms differ from all other coelomates (except for chordates) in their embryonic development. Very early in this development, a ball of cells called a blastula develops an infolding called a blastopore, which eventually reaches the other side of the embryo and forms the digestive tract. If the blastopore forms a mouth, the embryo is a called a protostome, meaning that the mouth (stoma) forms first (proto) after the anus. If the blastopore forms an anus, it is called a deuterostome, meaning that the mouth (stoma) forms second (deutero) after the anus. Echinoderm embryos are deuterostomes. This difference in development is so fundamental that protostomes and deuterostomes are thought to have diverged before any other branchings that led to the modern...