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EXPLAIN DIAGONOSTIC FEATURES OF CHORDATES
1.Single, hollow nerve cord beneath dorsal surface; in vertebrates, it differentiates into brain and spinal cord.
    2. Notochord: flexible rod on the dorsal side of gut, present at one stage in all chordates; displaced in vertebrates by vertebral column that forms around the nerve cord.
    3. Pharyngeal slits (pouches) connect pharynx (between mouth and esophagus) with  gills in sharks, fish; present in terrestrial animal embryos but disappear later except Eustachian tube (connecting throat and middle ear).
    4. Postanal tail extends beyond anus; present at least in embryo; regresses into tail bone in humans.
    5. Segmentation: reflected in arrangement of muscles (somites) and in vertebral column (both mesoderm).


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