The Adelochorda fall into two groups. The first, the Class Enteropneusta, is composed of worm-like, simply-organised burrowing marine animals such as Balanoglossus, Glossobalanus, Ptychodera, and Saccoglossus. The second group, the Class Pterobranchia, includes only three genera, Cephalodiscus, Rhabdopleura, and Atubaria. These, too, live in the sea. They are colonial and sedentary. The two classes have in common the peculiar structure of the stomochord (presumed notochord), the division of the body into three regions (which are sometimes looked upon as representing three segments), and certain other features.
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