In the Cyclostomata[377] (e.g. _Petromyzon_) the ventral aorta (Fig. 194) is continued forwards from the heart as a single vessel to the fourth pair of gill-sacs, where it divides into right and left branches which extend as far as the anterior walls of the first pair of gill-sacs. Eight pairs of afferent branchial arteries arise from the ventral aorta and its two branches, of {330}which the first and last supply the anterior walls of the first pair of sacs and the posterior walls of the last pair respectively. Each of the remaining afferent vessels extends into an interbranchial septum, and supplies the gill-lamellae of the posterior wall of one sac and those of the anterior wall of the next sac behind. The corresponding efferent branchial vessels have a similar distribution, and unite dorsally to form a median dorsal aorta. Beneath the base of the skull the latter vessel divides into two branches which, after receiving the first pair of efferent branchial vessels, pursue a divergent course forwards, but subsequently converge and unite to form a "circulus cephalicus," as in Teleosts. From the cephalic circle are given off on each side (1) an "internal carotid" artery for the brain and eye; (2) an "external carotid" for the lateral and ventral walls of the head; and (3) a large ventral branch which supplies the lingual apparatus; while from the abdominal portion of the dorsal aorta are derived, first, a coeliaco-mesenteric artery for the liver and alimentary canal, and subsequently branches for the myotomes, kidneys, and the gonad. The terminal portion of the aorta then enters the tail and forms the caudal artery. [Illustration: FIG. 194.—Branchial arterial system of the Lamprey (_Petromyzon fluviatilis_). The ventral aorta and the afferent branchial arteries are seen on the left side, and the efferent branchial and dorsal aorta on the right (diagrammatic). _af.b.a_, _ef.b.a_, Afferent and efferent branchial arteries; _a.o_, auditory organ; _b.c_, branchial canal; _c.c_, cephalic circle; _cm.a_, coeliaco-mesenteric artery; _d.a_, dorsal aorta; _e_, eye; _ex.c_, "external carotid"; _h_, heart; _i.b.s_, interbranchial septum; _in.c_, internal carotid; _oph.a_, ophthalmic artery; _r.v.a_, _l.v.a_, right and left ventral aorta; _v.a_, median ventral aorta; _v.c_, "ventral carotid"; 1-7, gill-sacs. (Modified from Vogt and Yung.)] In Elasmobranchs[378] (e.g. _Mustelus antarcticus_) the undivided ventral aorta gives off five pairs of afferent branchial arteries which, on each side, ascend in succession the outer convex sides of the hyoid and first four branchial arches (Fig. 195).