notochord(n = noun.animal) - a flexible rodlike structure that forms the supporting axis of the body in the lowest chordates and lowest vertebrates and in embryos of higher vertebrates;
is a kind of back, backbone, rachis, spinal column, spine, vertebral column
has particulars: urochord

   An elastic cartilagelike rod which is developed beneath the medullary groove in the vertebrate embryo, and constitutes the primitive axial skeleton around which the centra of the vertebræ and the posterior part of the base of the skull are developed; the chorda dorsalis. See Illust. of Ectoderm. [1913 Webster]

notochord, n. a cartilaginous skeletal rod supporting the body in all embryo and some adult chordate animals.

Etymology:
Gk noton back + CHORD(2)