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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- Fall '10
- Biology, pharyngeal gill pouches