Vascular System of Bdellostoma dombeyt. 21 
while the remaining vessels from the median trunk are asym- 
metrical, the /e/¢ vessels arising anterior to the corresponding 
vessels on the right side. The afferent branchial arteries 
arising from the right and left branches of the aorta are also 
asymmetrically placed, corresponding to the asymmetry of 
the gills. The afferent vessels vary in length, the anterior 
vessels being longer than those in the posterior region. 
Those from the main trunk are about 1-1.5 cm., while those 
from the lateral branches are 1.5-3 cm., increasing from 
behind forwards. The most anterior vessel is always the 
longest. In size they are all about equal. The direction of 
the arteries is external and slightly upward in the arteries 
from the main trunk, and forward, upward, and outward in 
those from the lateral branches. Each afferent branchial 
artery terminates on the postero-external wall of the corre- 
sponding gill pouch, just below the external gill passage. 
The last afferent branchial artery of each side gives off a 
small branch a short distance from the gill. This branch 
‘possesses a lumen only at its origin, if at all. It soon becomes 
reduced to a slender string of connective tissue which 
becomes lost in the connective tissue around the “club- 
muscle.” Attached to this string is a small spheroidal body, 
apparently made up of fibrous and fatty tissue. (See Fig. 
X, XIV.) 
It may be remarked that in Bdel/lostoma forsteri this string 
may be traced from the branchial artery around to the dorsal 
aorta. The significance of these structures will be discussed 
Partie. 
BRANCHIAL CIRCULATION. 
(Pigss VITR Pe TIT) 
The gills of the Bdellostoma are lens-shaped pouches, com- 
pressed laterally, so as to be concave on the inner face and 
convex on the outer. The pouches are not circular in out- 
line, but more nearly elliptical, being elongated dorso-ven- 
trally. The gill, as a whole, has two faces and four borders— 
superior, inferior, anterior, and posterior. The anterior and 
posterior are usually indented so as to be slightly concave, 
instead of convex. ‘The afferent gill passage enters at the 
middle of the concave zzzer wall of the gill. (Fig. XIX.) 
9 
