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All animals of the phylum cordata have four distinct characteristics at some time in their life.
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One is the hollow dorsal nerve cord.
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This runs along the back of the organism and will develop into the central nervous system, which is the brain and spinal cord.
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Next is the noda cord, which is a flexible cartilage rod that may be present or absent in an adult organism, but occurs during all cordata embryological development.
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It provides support and also runs along the back of an organism.
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We have next pharyngeal gillslets, which are openings along the pharynx.
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They are used for filter feeding in invertebrate chordates, gills, invertebrate fish, and they close in tetrapods, which is why humans don't have openings...