Vascular System of Bdellostoma dombeyt, 27 
XV, XVIII, XX, s). The somatic arteries, as a rule, alter- 
nate with the somatic vezzs on each side. (Fig. XII.) 
Those of the right and left sides are sometimes in pairs, 
opposite each other, but usually alternate, always corre- 
sponding to the arrangement of the myotomes on each side. 
The somatic arteries from the anterior dorsal aorta arise 
independently. Those from the fosterzory dorsal aorta usually 
arise by a short trunk in common with one or more renal 
branches (Fig. XII). Each somatic artery passes above the 
mesonephros, supplies it with one or more twigs, and then 
divides into two branches. (See Fig. XV.) The dorsal 
branch passes directly upward beside the notochord, supply- 
ing the lateral trunk muscles, and sending branches into the 
neural canal to supply the spinal cord. Then it passes up in 
the median longitudinal septum above the neural canal. It 
_ passes out to the skin of the dorsal region through the sheet 
of connective tissue which fastens the skin to the body wall. 
The ventral branches of the somatic arteries correspond to 
the zztercostal vessels of the higher vertebrates. (Figs. XII, 
XV, ic.) They pass outward along the roof and sides of the 
peritoneal cavity, between the lateral trunk muscles and 
the peritoneum. They pass along the septa between the 
myotomes, giving off many minute twigs to the adjoining 
muscles, (see. Pigs. XII; XV,-<1c.)  Hach,“intercostal”’ 
passes along with (anterior to) the corresponding intercostal 
nerve (sn). After passing between two slime glands, which 
it supplies, each intercostal vessel gives off irregular branches 
to the ventral rectus trunk muscle in the vicinity. The end 
piece of each vessel passes out into the skin in the neigh- 
borhood of the slime glands, and is distributed to the integu- 
ment of the ventral and lateral regions of the body. 
(f) Renal branches are supplied segmentally to the mesone- 
phros along each side of the aorta. A portion of these are 
shown in Figure XII, (r). (cfr. also Figs. XVIII and XX.) 
These renal branches supply the segmental glomeruli along 
the inner wall of the mesonephros. The glomeruli corre- 
spond to the myotomes only in a general way, and not 
exactly. Usually two glomerular vessels, and one or more 
twigs to the dorsal and inner walls of the mesonephros, arise 
from a common trunk with a somatic branch. There is 
I5 
