11.1.2 11.12: Echinoderms are vertebrates’ closest invertebrate relatives and include sea stars, sea urchins, and sand dollars.

Appearances can be deceiving. Sea stars and other members of the phylum Echinodermata live in the oceans, are radially symmetrical (as adults), and are covered by spiny plates. And yet, because they are deuterostomes (their gut forms from back to front), it turns out that they are more closely related to humans than to any other invertebrate group. Echinoderms stand as an example of how classifying organisms based on evolutionary relatedness—rather than on a comparison of observable physical traits—can reveal a great deal about the force of natural selection and its dramatic effects on body form in adapting populations of organisms to their environment.

The echinoderms include about 6,000 species of marine animals, most of which are enclosed by a hard skeleton of spiny plates (FIGURE 11-19). Adult sea stars (starfishes and brittle stars), probably the most recognizable echinoderms, have five or more appendages evenly distributed around their body circumference. They move with equal ease in any direction, and their sensory organs are distributed around their circumference. Adult sea urchins and sand dollars don’t have projecting arms, but they also are radially symmetrical.

Although radial symmetry is characteristic of adult echinoderms, their larvae are bilaterally symmetrical. Bilaterally symmetrical larvae are evidence that echinoderms evolved from bilaterally symmetrical ancestors, and their radial symmetry as adults is an adaptation associated with their mode of locomotion and feeding specializations.

Echinoderms do not have a brain. They have a nervous system that consists of a central ring of nerves, with branches that extend into each of their appendages and help them gain information about and respond to their environment. Echinoderms creep on little tube feet that are extensions of an internal system of water-filled canals radiating throughout the body. Tube feet carpet the undersides of sea stars’ arms and extend up and around the bodies of sea urchins and sand dollars, extending and contracting in waves to grasp and release the surface on which they move as the animal glides along.

Although each tube foot is tiny, there are thousands of them, and their combined force allows a sea star to pull the two shells of a clam or mussel apart. Once it has the shells apart, an extraordinary and unexpected thing happens. The sea star pushes its stomach out through its mouth and inserts it into the opened shells. The stomach then secretes digestive enzymes that break down the clam or mussel tissue, and the sea star absorbs the soup that results. Sea urchins feed on algae, scraping them loose from rocks with sharp tooth-like surfaces made of calcium. Sand dollars capture floating particles of algae and other organic matter by trapping them in streams of mucus, which are moved toward their mouth by beating cilia.