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The Class Ophiuroidea - related by a common ancestor...

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The Class Ophiuroidea The class Ophiuroidea consists of approximately 2,000 species of brittle stars. Brittle stars have a central disk from which long, flexible arms radiate. These long arms allow them to move rapidly. Phylum Hemichordata The Phylum Hemichordata includes about 90 species of acorn worms. Some acorn worms living on tidal mud flats have a proboscis, a collar, and a trunk. The dorsal nerve cord in the collar and trunk resembles the nerve cord of chordates. The pharynx below the collar has gill slits. Larva of hemichordates resembles the larva of echinoderms. These facts cause some scientists to believe echinoderms and hemichordates share a common ancestor and that hemichordates and chordates are
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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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