Notochord

Notochord, the cartilaginous rod which lies under the main nervous system, and is the primitive ' element of the skeleton,"in the phylum of animals known as the Chordata (q.v.). It extends typically from one end of the body to the other as a cylindrical rod tapering to a point in either direction; as such it occurs in the remarkable Amphioxus (q.v.) or Lancelet. In the Ascidians or Tunicates it is restricted to a short mass underlying the nerve cord in the caudal or tail section of the body. In the vertebrata, on the other hand, the skeleton is developed and encroaches upon the continuity of the notochord; it completely atrophies in the skull except in the sturgeon and the lampreys. It may persist as a series of soft disconnected masses between the bones of the back-bone or vertebral column, as in fish, or may be entirely lost. The Chordata form a phylum of the animal kingdom based upon this structure; it is divided into three sub-phyla: (1) the Urochorda, including the Ascidians, in which the notochord is restricted to the caudal section; (2) the Cephalochorda, comprising only Amphioxus, in which it extends throughout the body; and. (3) the Vertebrata, including the fish, reptiles, amphibia, birds, and mammals in which the notochord is lost wholly or in part, owing to the development of a body skeleton. A structure in Balanoglossus (q.v.) has been identified as a notochord, and this group of worms therefore included among the Chordata; this view, however, is not now accepted.