A New Look at Some Old Animals

When teaching an introductory zoology course, it is always entertaining to show students some specimens of the only described species of placozoans—Trichoplax adhaerens, a tiny, simple, nearly worldwide marine organism (Figure 1) [1,2]. The inevitable disbelief—those are animals?—leads naturally to enumerating the shared derived features of all animals (Box 1) and in turn to discussing the relationships among the various groups at the base of the animal or “metazoan” tree of life. These early diverging or “basal” groups include bilaterians, the bilaterally symmetric forms that most students recognize as animals—worms, flies, mice, and many more. Most students are also familiar with cnidarians—corals, anemones, jellyfish, hydroids—and perhaps even with sponges, known for their soft and porous skeletons. Less familiar are the ctenophores—comb jellyfish—and the enigmatic placozoans. How these five groups fit together at the root of the metazoan tree is a matter of intense debate and considerable study. Indeed, the ordering of the divergence of these basal groups affects our inferences of the features of the common ancestor of all animals. This in turn influences our understanding of the evolution of all animal characteristics, whether molecular, physiological, or morphological.