The bilaterians make up a large monophyletic group embracing all animals other than ctenophores, sponges, placozoans, cnidarians, and a few poorly known groups of parasitic animals (see Table 30.1). Some of the traits that support the monophyly of bilaterians are the presence of three distinct cell layers in embryos (triploblasty) and the presence of at least seven Hox genes (see Chapter 19). Although bilateral symmetry is often viewed as a synapomorphy of bilaterians (and the trait gives the group its name), some groups of cnidarians are also bilaterally symmetrical. Recent studies have shown that the genetic basis of bilateral symmetry in bilaterians and in those cnidarian groups that have this trait is similar, so this feature may have been present in the ancestor of both groups.

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Bilaterian animals can be divided into the two major clades mentioned earlier, the protostomes and the deuterostomes. These two groups, as well as the remaining animal groups we will discuss below, have been diversifying separately for at least 700 million years. We will describe the protostomes in Chapter 31 and the deuterostomes in Chapter 32.

The remainder of this chapter describes those animal groups that are not bilaterians. The ctenophores, sponges, and placozoans have weakly differentiated tissue layers or even undifferentiated layers, and two of these groups (sponges and placozoans) also lack a nervous system. The remaining animals (those that don’t fall into any of the groups mentioned thus far) are called eumetazoans. Eumetazoans typically have some form of body symmetry, a gut, a nervous system, and tissues organized into distinct organs (although there have been secondary losses of some of these features in some eumetazoans).