Unformatted text preview: related by a common ancestor. Hemichordates are classified into three classes, two with living members, one consisting of only fossil forms. The Enteropneusta includes the acorn worms; the Pterobranchia includes 20 species of colonial bottom dwelling worms. The fossil (extinct) group, the Graptolithina (graptolites), are common fossils in the Ordovician and Silurian. They have recently been placed in this phylum. Vertebrate Chordates The vertebrates comprise a large group of chordates, and are subdivided into seven classes (3 classes of fish, amphibians, reptiles, birds, and mammals). Vertebrates have an internal skeleton of cartilage or bone, with vertebrae surrounding the dorsal nerve cord....
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- Fall '10
- Biology, Hemichordata, Phylum Hemichordata, Class Ophiuroidea