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The genome is largely non-redundant, making it very comparable to vertebrates, but without the complexity. For example, 200 to 700 chemosensory genes were found that lacked introns, a feature typical of vertebrates. Thus the sea urchin genome provides a comparison to our own and those of other deuterostomes, the larger group to which both echinoderms and humans belong. Sea urchins are also the closest living relative to chordates. Using the strictest measure, the purple sea urchin and humans share 7,700 genes. Many of these genes are involved in sensing the environment, a fact surprising for an animal lacking a head structure.