Having introduced diversity within protostome animals, we now turn our attention to the other great bilaterian group, the Deuterostomia. Deuterostome animals (Fig. 44.25) include three major phyla: Hemichordata (acorn worms), Echinodermata (sea urchins and sea stars), and Chordata (vertebrates and closely related invertebrate animals such as sea squirts).

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Let’s begin with the deuterostomes most distantly related to the vertebrates, the hemichordates and echinoderms. Hemichordates include acorn worms, about 75 species of wormlike animals that move through seafloor sediments in search of food particles. Pterobranchs, which are also hemichordates, consist of about a dozen species of animals that attach to the seafloor and use tentacles to filter food from seawater (Fig. 44.26). Hemichordates all have a mouth on an elongate protuberance called a proboscis that connects to the digestive tract by a tube called the pharynx, which contains a number of vertical openings called pharyngeal slits separated by stiff rods of protein. They also have a dorsal nerve cord. Additional characters that relate hemichordates to other deuterostomes are apparent in embryological and larval development. Nonetheless, adult hemichordates have body plans quite distinct from those of chordates and echinoderms.

Another major group of deuterostomes is the echinoderms, among the most distinctive of all animal phyla. Echinoderms include sea stars and sea urchins (including sand dollars), as well as brittle stars, sea cucumbers, and sea lilies—all of which share a unique fivefold symmetry on top of their basic bilaterian organization (Fig. 44.27). Echinoderms form distinctive skeletons made of interlocking plates of porous calcite, a form of calcium carbonate. This is why they generally feel hard and stony, and why they are well protected from most predators. The third unique feature of echinoderms is the water vascular system, a series of fluid-filled canals that permits bulk flow of oxygen and nutrients. Tube feet, small projections of the water vascular system that extend outward from the body surface, allow echinoderms to crawl, sense their environment, capture food, and breathe. Sea stars can use the tube feet on their powerful arms to pull open the valves of clams. About 7000 echinoderm species reside in present-day oceans, but the fossil record suggests that this phylum was more diverse in the past.

Zoologists have long known that echinoderm and hemichordate larvae share many features. Molecular sequence data now confirm that echinoderms are the closest relatives of hemichordates, with chordates a sister group to the group containing echinoderms and hemichordates (see Fig. 44.25).