notochord [key], in biology, supporting rod running most of the length of animals of the phylum Chordata and present at varying times in the life cycle. Composed of large cells packed within a firm connective tissue sheath, the notochord lies between the neural tube (spinal cord) and the gut. The division of the phylum Chordata into subphyla is based on the structure of the notochord and the time of life in which it is present: In the subphylum Urochordata (tunicates) the notochord characterizes the larval, swimming, stage of the animals and does not extend into the head; in the subphylum Cephalochordata (lancelets) the notochord extends to the extreme tip of the head in both young and adults; and in the subphylum Vertebrata the notochord becomes surrounded by skeletal vertebrae during embryonic development—in higher vertebrates it is present in the early embryo only and is later completely replaced by the vertebrae.
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