Definition: 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.
