It may surprise you to learn that both you and a sea urchin are deuterostomes. Adult sea stars, sea urchins, and sea cucumbers—the most familiar echinoderms—look so different from adult vertebrates (fishes, frogs, lizards, birds, and mammals) that it may be difficult to believe all these animals are closely related. The evidence that all deuterostomes share a common ancestor that is not shared with the protostomes includes early developmental patterns and phylogenetic analysis of gene sequences, factors that are not apparent in the forms of the adult animals.


The three major clades of deuterostomes are the echinoderms, the hemichordates, and the chordates. The common ancestry of these groups is supported by early developmental similarities and by phylogenetic analyses of DNA sequences. The earliest deuterostomes had bilateral symmetry, pharyngeal slits, and a segmented body.

Echinoderms are characterized by pentaradial symmetry, but they are bilaterally symmetrical as larvae. Echinoderms also have an internal skeleton of calcified plates. Also unique to this group is a water vascular system, a network of water-filled canals leading to extensions called tube feet that functions in gas exchange, locomotion, and feeding.

Hemichordates have a bilaterally symmetrical body divided into three parts: proboscis, collar, and trunk.

Chordates are characterized by a dorsal hollow nerve chord, a post-anal tail, and a dorsal supporting rod called a notochord at some point during the life cycle. Specialized structures for support (a vertebral column), locomotion (such as fins), and feeding (jaws and teeth) evolved among the vertebrates.

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Textbook Reference: Concept 23.5 Deuterostomes Include Echinoderms, Hemichordates, and Chordates