Bilaterally symmetrical animals (e.g., humans, lizards, grasshoppers, squid) can be broken up into two major groups: deuterostomes and protostomes. Chordates (the phylum humans and lizards belong to) are deuterostomes, but there actually aren't too many other deuterostome phyla; besides Chordata, there's only Hemichordata (acorn worms) and Echinodermata (sea stars, brittle stars, sea urchins, etc.). The vast majority of animal phyla are protostomes (e.g., Arthropoda, Mollusca, Annelida, Nematoda, Rotifera).
A recent Nature paper (Bourlat et al. 2006) has proposed that Xenoturbella bocki, a worm with no body cavity, no digestive tract (even though it feeds on bivalves), and no major organs, is actually a deuterostome. In fact, they've proposed that it belongs in its own brand new phylum of deuterostome, Xenoturbellida. PZ Myers has the full story, complete with figures. It's not every day that a new deuterostome phylum gets proposed; go and enjoy!
Bourlat SJ, Juliusdottir T, Lowe CJ, Freeman R, Aronowicz J, Kirschner M, Lander ES, Thorndyke M, Nakano H, Kohn AB, Heyland A, Moroz LL, Copley RR, Telford MJ. (2006) Deuterostome phylogeny reveals monophyletic chordates and the new phylum Xenoturbellida. Nature. 444(7115):85-8.
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