igS PHYSIOLOGY OF THE INVERTEBRATA. 



anterior extremity, which is prolonged so as to form a short 

 aorta, which' has at its base a valvular apparatus. This 

 apparatus opens and closes at regular intervals. On leaving 

 the open end of the aorta, the blood flows in two different 

 directions, one part anteriorly, the other posteriorly. The 

 considerable quantity of blood conducted to the anterior part 

 of the body is seen to flow down the sides of the head, partly 

 supplying its several appendages, partly running straight 

 back along its ventral side to the region of the adductor 

 muscle of the shell. Here the blood enters the valves, being 

 received within the complicated system of canals occurring 

 between their two lamellae. The other principal arterial cur- 

 rent is seen running from the heart backwards along the 

 dorsal side of the trunk, immediately above the intestine; 

 and, on reaching the tail, it bends round and flows anteriorly 

 along the ventral side, sending off, in each segment, lateral 

 currents to the branchial legs. The blood thus conducted to 

 the various parts of the body and shell returns to the heart 

 by two different ways. The considerable quantity of blood 

 introduced within the canal-system of the shell is at last 

 received by two longitudinal sinuses passing along its dorsal 

 side, the anterior rather short, the posterior occupying the 

 greater part of the dorsal line. In the anterior sinus the 

 blood flows backward, in the posterior, forward ; the two 

 currents meeting at the place where the body is connected 

 with the shell dorsally. Here both currents suddenly bend 

 down, the one on the anterior, the other on the posterior side, 

 and pour out the blood into the pericardial sinus, whence it 

 passes into the heart through the lateral valvular openings. 

 The remaining part of the blood, introduced into the trunk 

 and the tail, is at last received within a large sinus, occupy- 

 ing the upper part of the dorsal side of the trunk and 

 divided from the arterial dorsal sinus by a longitudinal 

 ligament extending from the lower side of the heart to the 

 tail. This blood-sinus is apparently fed in each segment of 

 the trunk by a pair of ascending currents from the branchial 



