20 Cincinnati Society of Natural History. 
ARTERIAL SYSTEM. 
The ventral aorta (Figs. X, XVIII, Av.) consists of a 
principal median trunk, which divides anteriorly into a pair 
of lateral branches. ‘The main trunk lies in the median line, 
just above the ventral body wall and below the pharynx. 
The length is about 3cm. The lateral branches are a little 
longer. It extends from the anterior end of the ventricle to 
the posterior extremity of the “‘club-muscle.” It lies between 
the last six or seven pairs of gills, and is imbedded in the 
peculiar brownish fatty connective tissue characteristic of 
that region. The median trunk of the aorta is nearly cylin- 
drical in shape, but narrows slightly in caliber anteriorly. 
Posteriorly, at its junction with the ventricle, it is suddenly 
constricted. The relatively small opening from the ventricle 
is guarded by a pair of strong semi-lunar valves, as previously 
described. The short expanded portion of the aorta just 
beyond the constriction and behind the first gill branches 
represents the du/bus aorte. This portion of the aorta is in 
contact ventrally with the dorsal surface of the anterior lobe 
of the liver. 
The ventral aorta divides anteriorly into two branches (right 
and left) which pass forward, upward, and outward, along the 
dorso-lateral aspect of the posterior end of the club-muscle. 
The posterior end of the club-muscle, with reference to the 
point of branching, is variable, on account of the mobility of 
the former. The usual position is shown in Fig. X. But 
the muscle may be drawn forward, or even back below the 
point of division. 
The afferent branchial arteries (Figs. X, XVIII, af. br.) 
rise on either side from the ventral aorta and its right and 
left branches. There are usually six or seven pairs arising 
from the median trunk, and from three to six pairs from the 
branches. Sometimes there is one more on one side than on 
the other, corresponding to the asymmetrical occurrence of 
the gills. The six or seven pairs from the median trunk are 
never symmetrically disposed. The last afferent branchial 
artery on the right side is always posterior to the correspond- 
ing left vessel. (See Fig. X.) The two pairs lying next 
anterior to the last are situated nearly opposite to each other, 
8 
