Early in the history of biology, taxonomists such as Carolus Linnaeus recognized that for all their diversity, animals can be sorted into a limited number of distinctive body plans, each group today formally called a phylum. The problem for biologists has long been how to understand the evolutionary relationships among animal phyla, still an active field of research.

Nineteenth-century scientists recognized that animals vary in their structural complexity. Sponges, for example, have only a few cell types, and those cells are not organized into tightly coordinated tissues or organs. Jellyfish and sea anemones have simple tissues but do not have complex organs, whereas crabs and alligators have complex organs specialized for movement, behavior, digestion, and gas exchange.

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Early biologists could also see that animals differ in the symmetry of their bodies (Fig. 44.2). Sponges are often irregular in form, with no plane of symmetry. In contrast, jellyfish and sea anemones display radial symmetry, meaning that their bodies have an axis that runs from mouth to base with many planes of symmetry through this axis (Fig. 44.2a). This organization allows jellyfish to move up and down in the water column by flexing muscles around their bell-like bodies, and permits sea anemones and corals to wave their ring of food-gathering tentacles in all directions at once. Most animals, however, show bilateral symmetry: Their bodies have a distinct head and tail, front and back, and right and left, with a single plane of symmetry running between right and left at the midline (Fig. 44.2b). Bilateral symmetry enables animals to move in one horizontal direction to capture prey, find shelter, and escape from enemies. It also allows the development of specialized sensory organs at the front end for guidance (Chapter 36), and specialized appendages along both sides for locomotion, grasping, or defense.

Within the bilaterian group, however, understanding evolutionary relationships proved more difficult to early taxonomists. Some biologists pointed to the presence or absence of a cavity, or coelom, surrounding the gut, dividing bilaterians into three groups: those without a body cavity (acoelomates) and those with a body cavity (coelomates and pseudocoelomates, which differ in the embryonic origin of the cells lining the cavity). These body plans are shown in Fig. 44.3. A body cavity cushions the internal organs against hard blows to the body and enables the body to turn without twisting these organs. It also allows internal organs like the stomach to expand, enhancing digestive function.

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New insights became possible with the advent of microscopes that enabled the direct study of development from its earliest stages. One important observation was that some animals that look very different as adults share patterns of early embryological development. For example, adult sea stars and catfish have few morphological features in common, but their embryos show a number of key similarities, including the number of embryonic tissue layers they develop and the way that the early cell divisions occur. In the phylum Cnidaria, which includes jellyfish and sea anemones, the embryo has two germ layers, the endoderm and the ectoderm, from which the adult tissues develop. These animals are therefore called diploblastic (Fig. 44.4). Bilaterally symmetrical animals, the Bilateria, are triploblastic, with a third germ layer, the mesoderm, lying between the endoderm and ectoderm, which develops into muscles and connective tissues (Fig. 44.4).

Comparative embryology also enabled biologists to divide bilaterian animals into the two groups shown in Fig. 44.5, which are called protostomes (from the Greek for “first mouth”) and deuterostomes (from the Greek for “second mouth”). In protostomes, the earliest-forming opening to the internal cavity of the developing embryo, called the blastopore, becomes the mouth. In deuterostomes, the blastopore becomes the anus. However, only in the age of molecular sequence comparisons have the relationships among phyla within each of these two groups become clear.