A grand division of the animal kingdom, intermediate, in some respects, between the invertebrates and vertebrates, and in modern classifications considered a subphylum of the vertebrates; called also urochordata
. They were formerly classed with acephalous mollusks. The body is usually covered with a firm external tunic, consisting in part of cellulose, and having two openings, one for the entrance and one for the exit of water. The pharynx is usually dilated in the form of a sac, pierced by several series of ciliated slits, and serves as a gill. [1913 Webster
" Most of the species when mature are firmly attached to foreign substances, but have free-swimming larvæ which are furnished with an elongated tail and somewhat resemble a tadpole. In this state the larva has a urochord and certain other structures resembling some embryonic vertebrates. See Ascidian, Doliolum, Salpa, Urochord, and Illust. of Social ascidian, under Social."