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3 History of major animal groups The simplest living phyla may have no direct relationships to other animals (Fig. 2), but they indicate the versatility of multicellular design. For example, members of the Porifera (sponges) contain several differentiated cell types that form body walls, but their architecture is limited to folding these walls into chambers that are sometimes highly convoluted. Members of the Cnidaria (also termed Coelenterata, includ- ing sea anemones, corals, and jellyfish) have two cellular body layers, separated by a gelatinous layer; the inner tissue forms a digestive cavity. Flatworms (Platyhelminthes) and their allies have solid bod- ies, with both external (ectoderm) and internal (endoderm) tissue layers separated by a third cellular layer (mesoderm); there is a gut lined with endoderm. All phylogenetic evidence suggests that a flatwormlike form gave rise to the remaining phyla, which possess body cavities (other than a gut) in which reproductive and other organs can be sequestered. In about 15 of these phyla, this cavity (the coelom) is lined by mesoderm and usually functions as a hydrostatic skeleton. The coelomates constitute all of the familiar, larger-bodied animals and are commonly grouped into two main divisions (Fig. 2). In one group, the body was originally segmented, as in annelid worms or arthropods, although the segmentation may be reduced or lost in some branches. In the other group, the coelom is divided into two or three regions; these include the Enchinodermata (starfish and sea urchins), Tunicata (sea squirts), and Chordata (including humans). Additionally, there are coelomate phyla that cannot yet be assigned to either of these groups; some, such as the Brachiopoda, Phoronida, and Bryozoa, display an enigmatic mixture of traits that otherwise characterize one or the other of the main groups. In about 10 phyla (not shown in Fig. 2), the body cavity is not a true coe- lom, but lies between endoderm and mesoderm and is termed Fig. 3: Marine fossil record of major animal groups during the last 680 million years. The width of the spindles is proportional to the number of families present. Note the shifting of domi- nance from (a) trilobites and other forms during the early Paleozoic to (b) articulate brachiopods, crinoids, and others in the late Paleozoic, and then to (c) gastropod and bivalve mollusks. (Credit: J. J. Sepkoski, Jr.) Animal Evolution (continued) + ward ' s science