Chapter 33, Problem 1CQ

Fierce debate centers on whether ctenophores or sponges are the earliest-diverging animals. Choanoflagellates, the closest living relatives to animals, appear very similar to the choanocyte cells of sponges. In addition, sponges lack true tissues, although they have distinct types of cells. In contrast, all other animals possess one or more types of tissues. Both the Cnidaria and the Ctenophora are radially symmetric and, until very recently, both were thought to have only two germ layers, endoderm and ectoderm. All other animals are bilaterally symmetric and have three germ layers. Draw an animal phylogenetic tree that illustrated this viewpoint, including Cnidaria and Ctenphora as part of the same phyla, and label the critical innovations.