The Hypobranchial Arteries. (Figs. 3 and 5.)

The hypobranchial arteries arise from the ventral ends of the efferent loops and form a complicated system of vessels on the ventral wall of the pharynx. They supply the heart by the coronary arteries, and are joined to the subclavian arteries (s.cl.a.) by the coracoid arteries (cor.a.). In Typhlonarke the first hypobranchial artery arises from the first efferent loop and runs medio-posteriorly, ventral to the afferent arteries. It divides into two arteries, both supplying the thyroid gland. The second, third and even the fourth hypobranchial arteries may unite and run backwards as the lateral hypobranchial artery (l.h., Figs. 3 and 5). The second does not always join the lateral hypobranchial. The first gives rise to a superficial artery (Fig. 5, s.h. 1) laterally which divides to supply the ventral musculature of the gill flaps and the pharynx. A similar one arises from the second hypobranchial (s.h. 2), but the third superficial artery (s.h. 3) arises, not from the third hypobranchial but from the lateral hypobranchial. It supplies the last three gill flaps. The coronary artery (Fig. 3, C, cn.a.) arises from the median side of the lateral hypobranchial and runs along the common stem of the three posterior afferent arteries. On the left side, the coronary artery, before reaching the ventral aorta gives off a dorsal pericardial branch (d.p.). On the ventral aorta it turns posteriorly at right angles and runs along the conus to the ventricle. On this it divides into a ventral coronary artery (v.c.a.) and a dorsal coronary artery (d.c.a.). The lateral hypobranchial artery gives a branch to the ventral wall of the pericardium, but there is no posterior coronary artery. The lateral hypobranchial continues backwards as the coracoid artery (cor., Figs. 3 and 5), which passes dorsal to the coracoid cartilage and joins the subclavian artery.

In Torpedo (Fig. 3, B) as in Typhlonarke the first hypobranchial is not connected to the second, but it is the second which supplies the thyroid gland. The second hypobranchial runs towards the ventral aorta, sends two branches anteriorly, and then is joined to the third hypobranchial by a commissural vessel (c.h.a.). This was found on one side only. The coronaries arise one from the commissural hypobranchial and the other from the base of a sqxiare formed by the arteries on the ventral aorta. The third hypobranchial sweeps back as the coracoid artery dorsal to the coracoid cartilage. Owing to the posterior part of the specimen being destroyed it was not possible to locate a connection with the subclavian artery, though one no doubt exists.

In Typhlonarke the lateral hypobranchial artery is ventral to the afferent arteries. This is not the case in the sharks, where this vessel is dorsal to the afferent arteries and median hypobranchials are