Vascular System of Bdellostoma dombeyt. 25 
Me 
tively large lateral branch is given off on each side which 
runs forward and supplies the muscles in the lateral region 
of the head. In one instance I traced this vessel (on the left 
side), forward under the lateral trunk muscles just outside 
the skull wall, under the hyoid arch, and out into the orbital 
region, sending a branch out to the skin, in the fibrous 
band of connective tissue just below the eye. ‘The main 
trunk of the internal carotid joins the corresponding vessel 
of the opposite side in the median line just below the noto- 
chord and in front of the supra-pharyngeal plate. (See 
Fig. XX.) 
The vertebralis impar (v), formed by the junction of the 
internal carotids, runs forward a short distance in the median 
line beneath the notochord, giving off branches to the brain 
and cranial wall. Passing below the cranium, just behind 
the pituitary sac, it divides into two lateral branches, right and 
left, which run forward on each side of the base of the 
cranium to the nasal and anterior head region. 
The anterior dorsal aorta (Figs. XVIII, XIX, XX, Aa) is 
that portion of the median dorsal aorta which lies in, and in 
front of, the gill region. Posteriorly, it begins with the most 
posterior point of connection with the lateral carotids. It 
lies immediately above the pharynx, and below the noto- 
chord. In the gill region, the anterior dorsal aorta gives off 
four or five pairs of branches to the body wall (somatic 
branches). ‘The posterior, three pairs of these usually pierce 
the’ overlying “‘‘gill constrictor’’ muscles. In the anterior 
gill region, the aorta curves slightly to the right, and con- 
tinues forward beneath the right side of the notochord. In 
one instance it turned first to the left, ran forward two or 
three somites, then passed over to the right and forward as 
usual. 
Anterior to the gill region it gives off in its course seven 
or eight pairs of somatic branches to the adjacent segments 
of the body wall. In general, an artery is given off to each 
alternate myoseptum on each side. Each somatic vessel 
divides into two branches, dorsal and ventral, whose course 
is the same as that of those to be described in the abdominal 
region (see Fig. XV). There are some variations in the 
distribution of the branches of the anterior dorsal aorta, 
rg 
