The BLOOD SYSTEM of Amphioxus, although as simple as that of a Chaetopod worm, is undoubtedly laid down on the Vertebrate plan—even though there is no distinct heart and the vessels are few and of simple structure. Capillary networks are formed in some places, but the colourless blood also extends into many lacunae or lymph spaces, such as those around the fin-rays and in the metapleura. As in a typical lower Vertebrate, there is a contractile ventral vessel (the ventral or branchial aorta, Fig. 77, _v.ao_) running forwards under the alimentary canal to the pharynx, and giving off on each side afferent branchial vessels, which pass up the primary branchial bars and give off branches joining the vessels in the secondary bars. These latter do not communicate directly with the ventral aorta, but the vessels in all the branchial bars open dorsally by efferent branchial vessels into the paired dorsal aortae (Fig. 77, _d.ao_), which run backwards along the top of the pharynx, one at each side of the epipharyngeal groove. In the vessels of the branchial bars and their connectives the blood is aerated by the current of water passing through the gill-slits, and so reaches the dorsal aortae in a purified condition. The right-hand dorsal aorta is continued forward further into the snout than its fellow of the other side, and is dilated at its extremity (Fig. 77). At the posterior end of the pharynx the paired dorsal aortae unite to form the median dorsal aorta {125}which runs backwards, lying between notochord and alimentary canal. This vessel gives off branches to the wall of the intestine, and these break up into capillary networks (Fig. 77, _cp_), from which the blood is collected by the median sub-intestinal vein. This then flows forwards to pass by the hepatic portal vein to the ventral edge of the saccular liver, in the wall of which it is distributed in a capillary network. The blood is collected on the dorsal edge of the liver by the hepatic vein, which runs posteriorly and then turns downwards and forwards to become continuous with the posterior end of the ventral aorta or "heart." [Illustration: FIG. 77.—Diagram of the vascular system of Amphioxus. _af.br.a_, Afferent branchial arteries; _af.br.a′_, similar vessels of the secondary (tongue) bars; _br.cl_, gill-slits; _cp_, intestinal capillaries; _d.ao_, paired dorsal aortae; _d.ao′_, median dorsal aorta; _ef.br.a_, efferent branchial arteries; _hep.port.v_, hepatic portal vein; _hep.v_, hepatic vein; _int_, intestine; _lr_, liver; _ph_, pharynx; _s.int.v_, sub-intestinal vein; _v.ao_, ventral aorta. (From Parker and Haswell.)]