1. A flexible skeletal rod notochord that supports the posterior part of the body in some sea animals such as sea squirts.
2. The notochord (flexible rodlike structure) of an ascidian (marine animal with a saclike body and siphons through which water enters and leaves) or tunicate (marine animal with a saclike body), more conspicuous in the larva than in the adult and confined primarily to the caudal (tail or hind) region of the body.