Spencer—Zhe Blood Vessels of Ceratodus. 7 
is interesting to note that it is the larger of the two. In this respect the embryo 
Rana presents a feature characteristic of the adult Holocephali and Teleostei and of 
the embryo Pristiurus. 
The four epibranchial arteries formed by the union of the successive pairs of 
efferent vessels pass backwards and towards the middle line immediately beneath the 
vertebral column, and by their union give rise to the dorsal aorta. The epibranchial 
arteries unite in pairs—first, those of the first and second branchial arches, then those 
of the third and fourth (figs. 2, 3, &e.). By this union on each side two main arteries 
are formed which again unite to form the dorsal aorta which runs backwards 
immediately beneath the vertebral column until at the posterior end of the body 
where the hemal arches commence it is continued into the tail region as the caudal 
artery. 
C. Branches arising from the efferent branchial arteries and the epibranchial arteries. 
(a) From the anterior efferent artery of the first branchial arch (figs. 2, 3, 
and 6). 
Instead of ending blindly at the ventral termination of the first branchial 
hemibranch, the efferent vessel is continued downwards towards the ventral surface. 
It turns forwards and at the level of the posterior edge of the hyoid arch comes to 
lie close to the inferior jugular vein. At this point it divides into two. The first 
of these runs forwards into the muscles on the lower surface of the head and passes 
to the region in the floor of the mouth, where it gives a small branch which unites 
in the median line with one from the corresponding vessel of the opposite side. This 
vessel may be‘called the “nxgua/ artery and in its origin from the ventral end of the 
efferent vessel of the first branchial arch it is precisely similar to that of the lmgual 
artery as described by Marshall and Bles in the frog (figs. 2, 3, and 6, “xg. avt.). 
The second branch turns outwards and passes immediately dorsad of the inferior 
jugular vein, crossing to the posterior side of this (fig. 6). It then runs along the 
outer flattened surface of the ceratohyal beneath the opercular bone, giving off a 
few small branches to the muscles in this part. Then passing through a notch 
immediately beneath the 7th cranial nerve it turns sharply backwards and runs to 
the hyoidean hemibranch. In this it divides into two branches, one running 
downwards and the other upwards to join the small branch running across to the 
anterior efferent artery of the first branchial arch. 
This second vessel is the hyozdean artery (figs. 3 and 6, Zy. art). 
Thus, as in Teleostei, the hyoidean gill is supplied with arterial blood from the 
ventral end of the first branchial efferent artery. In Protopterus and Lepidosiren, 
