Before describing these latter, however, we must briefly allude to the highly remarkable group of the tunicates or sea squirts, one of which is shown in Fig. 64. They are sedentary creatures found attached to rocks or weeds on the sea floor, and in appearance they remind one rather of misshapen potatoes than of higher animals. They are in fact regarded by the fishermen who bring them to the surface as plants, and they were for long looked upon by zoologists as akin to the molluscs. The only definite external features of the tunicates are two apertures at the upper end, one in the centre and one somewhat on one side. The absence of any other definite external characters is due to the fact that the creature is enclosed in a mantle of cellulose. The central opening is the mouth, which leads into a large pharynx, the walls of which are perforated by numerous gill slits. This is surrounded by the mantle cavity, which connects with the outer water by means of the second pore. The gut is continued into a simple stomach and intestine, the latter bending back upon itself and opening into the bottom of the mantle cavity, as shown in the diagram in Fig. 63. In the adult animal there is no trace of the notochord, and only a remnant of the nerve cord; and there are either no special sense organs or only traces of these. On the other hand, the tunicates possess a centralised heart. They are hermaphrodite, and, very curiously, a number of forms multiply like corals, by a simple process of budding. [Illustration: FIG. 64.--A Tunicate. _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.