Phylum3 - pouches modified for various purposes in humans...

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Phylum Chordata The Phylum Chordata includes about 45,000 species that occupy nearly all environments. All chordates at sometime during their life history have: a notochord : a dorsal supporting rod located dorsally just below the nerve cord; it provides support and is replaced by the vertebral column in vertebrates a dorsal hollow nerve cord : a fluid-filled canal; spinal cord is protected by vertebrae pharyngeal gill pouches : openings that function in feeding, gas exchange, or both These features are seen only during embryonic development in most vertebrates. Not all chordates are vertebrates . In the invertebrate chordates, fish, and amphibian larvae, pharyngeal gill pouches become functioning gills. Terrestrial vertebrates have their
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Unformatted text preview: pouches modified for various purposes; in humans, the first pouches become the auditory tubes, the second become tonsils, and the third and fourth pairs become the thymus and parathyroid glands. Most chordates have an internal skeleton against which muscles work. Most have a postanal tail that extends beyond the anus; in some (like humans), this may only appear in embryos . The evolutionary origin of chordates remains a mystery, although biochemistry and comparative embryology indicates echinoderms and chordates share a common ancestry. Although scanty, fossil finds from the Cambrian suggest chordates were present in the Burgess Shale deposits....
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