426 
bulletin of the bureau of fisheries. 
ARTERIAL SYSTEM. 
The arterial system is not bilaterally symmetrical, so that a description of the arte- 
ries of each side will be necessary. 
The heart lies in the pericardium just anterior to the adductor muscle and mantle 
gland. It consists of a heavy walled ventricle and two thin walled auricles. (Fig. 17, h, 
pi. xlix.) The latter are elongated in the direction of the longitudinal axis of the body 
and are attached at one extremity to the tissue covering the retractor muscle and at the 
other to the adductor. They receive the blood from the gills through a short tube 
which lies external to the retractor muscle. (Fig. 17, t, pi. xlix, and text fig. 9, a.) 
The ventricle is a saddle-shaped structure into which the auricles open on either 
side. It gives off an anterior and a posterior aorta. The latter soon gives rise to a large 
branch which passes dorsally to the right of the rectum and enters the mantle. This 
artery divides into two equal branches at the posterior dorsal angle of the mantle, one 
branch going to the right mantle lobe, while the other goes to the left. 
The arteries of the left mantle are represented in figure 17, plate xlix, the right 
mantle lobe having been removed and its artery being therefore shown cut off. The 
mantle artery branches very profusely. It will be noted that there are two parallel 
arteries connected by numerous anastomoses. The outer and smaller of the two is dis- 
tributed to the edge of the mantle, the other branches mostly in the opposite direction, 
and supplies the greater part of the mantle. This posterior mantle artery meets and 
joins with a similar one from the anterior end of the body. 
The posterior aorta gives rise to a second branch, which is distributed to the rectum 
and mantle gland, then, bending abruptly ventrally, it enters the adductor muscle. A 
small branch continues over the anterior face of the adductor and goes to the region of 
the visceral ganglia and kidneys. (Fig. 18, pi. xlix.) 
The anterior aorta is much the larger of the two. On the right side (fig. 17, pi. xlix) 
it gives rise to five branches which go to the reproductive organ and liver. Three 
small branches go to the dorsal part of the mantle where they spread anteriorly and 
posteriorly in the midline. At the anterior end of the visceral mass the aorta gives 
off a branch which passes forward over the anterior retractor muscle. Three arteries 
arise from this branch; one to the outer palps, one to the middorsal line of the mantle, 
and one to the anterior adductor muscle. It then passes over the anterior adductor 
and at the extreme anterior end of the body divides into two equal branches, one of 
which goes to the right mantle lobe and the other to the left. These two branches 
join with the similar mantle arteries which arise from the posterior aorta. 
The aorta after giving off the artery, which has just been described as passing 
above the anterior retractor muscle, bends ventrally and divides into a number of arteries 
which are distributed to the inner palps and byssal apparatus and foot. Those which 
go to the bvssus are paired, right and left; but those to the left side are not represented 
in the figure. 
The arteries given off from the aorta on the left side of the body are represented in 
figure 18, plate xlix, and are three in number. All three are distributed to the digestive 
