It is clear that this course of the circulation agrees with that of a typical lower Vertebrate in all essential points:—(1) in having the main artery a dorsal aorta in which the blood flows backwards; (2) in having a ventral vessel representing the heart, and sending impure blood forwards to the respiratory region of the alimentary canal to be aerated; and (3) in having a hepatic portal system consisting of the capillaries of the liver, through which the blood from the intestinal wall has to pass before reaching the ventral vessel (heart). RENAL EXCRETORY functions have been attributed to various organs in Amphioxus, and it is quite possible that, in addition to the true nephridia which are now known, other tracts of tissue in the body may be able to eliminate nitrogenous waste matters. Such are certain clumps of columnar epithelial cells on the floor of the atrium, and the single pair of large brown atrio-coelomic funnels lying on the dorsal edge of the posterior end of the {126}pharynx (Fig. 71, _br.f_). There are, however, a large number (about 100 pairs) of minute nephridia, discovered (1890) by Weiss and by Boveri independently, lying at the sides of the dorsal coelomic canals above the pharynx, which must be regarded as the chief functional renal organs. These are bent tubules, partly glandular and partly ciliated, each giving off several caecal knobs (at first supposed to be open nephrostomes, one shown at each end of the tubule and three along its upper surface in Fig. 78), which project into the coelom, and opening by one nephridiopore (on the lower surface, and opposite a tongue bar of the pharynx) into the atrial cavity. The knobs, or closed nephrostomes, are surrounded by peculiar, slender, club-shaped tubular and flagellated cells—which Goodrich[113] has shown to correspond to the "solenocytes" in the nephridia of Polychaete worms (see Fig. 79). [Illustration: FIG. 78.—_Branchiostoma lanceolatum_. A nephridium of the left side with part of the wall of the pharynx, as seen alive, highly magnified. (From Willey, after Boveri.)]