A grand division of the animal kingdom, intermediate,
   in some respects, between the invertebrates and vertebrates, and by
   some writers united with the latter. 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.