_in.ap._, Opening leading to mouth; _ex.ap._, opening of peribranchial chamber.
From Dendy's _Outlines of Evolutionary Biology_ (Constable).]
[Illustration: FIG. 65.--Larva of Tunicate.
_n_, Nerve cord; _s_, sense organs; _m_, mouth; _kd_, pharynx; _vd_, alimentary canal; _ch_, notochord.]
Now the remarkable fact has been made out that the young tunicate (see Fig. 65) bears a most striking resemblance to the immature lancelet. It is a free-swimming, tadpole-like creature, and possesses a notochord and nerve cord and in general the same characters that we described for Amphioxus. It is only later that the creature settles down and assumes its final degenerate sedentary form. There can be no doubt that the tunicates have been derived from some lancelet-like form, but the course of their evolution has been unique. The type is the lost brother of the vertebrate family, who has chosen a distinctly downward path in life; yet who has come to no miserable end, but lives on, more or less successfully, in his lower social sphere.
The round-mouths, including the lampreys and the hag-fish, stand midway between the lancelet and the fishes, and therefore constitute for us an important group.