Fig. 1. Evolutionary relationships and germ layer composition of Cnidaria and Bilateria. (A) The topology of the tree summarizes the results of recent molecular phylogenetic analyses (Odorico and Miller, 1997; Schuchert, 1993). The phylum Cnidaria is an outgroup of the Bilateria, perhaps the sister group. The class Anthozoa is the sister of the Medusozoa (Odorico and Miller, 1997; Schuchert, 1993). Anthozoans exhibit only the polyp stage, while most medusozoans exhibit both polyp and medusa life stages. The Medusozoa comprises the members of the classes Hydrozoa, Cubozoa and Scyphozoa. Multiple independent scyphozoan lineages are depicted because a recent analysis suggests that the Scyphozoa may be paraphyletic (Collins, 2002). (B) The germ layer composition of representative taxa is indicated using diagrams of the cross-sectional anatomy. The anthozoan polyp is sectioned through the pharynx. Notice that the lumen of the pharynx is lined with ectoderm, while the outer surface of the pharynx is lined with endoderm. The hydrozoan polyp lacks a pharynx. Both animals exhibit two epithelial layers (endoderm and ectoderm). The hydrozoan medusa exhibits a third epithelial layer, the entocodon, that surrounds a coelom-like internal cavity, the subumbrellar cavity (Boero et al., 1998). Mesoderm lines the coelomic cavity of both protostomes and deuterostomes.