386 THE MUSSEL chap. 



which, by their insertion into the shell, give rise to the 

 line already seen. 



The cxlovie is reduced to a single ovoidal chamber, the 

 pericardium (Figs. 93 and 95, pc), lying in the dorsal region 

 of the body and containing the heart and part of the intes- 

 tine : it is lined by coelomic epithelium, and does not corre- 

 spond with the pericardial sinus of the crayfish, which is 

 a blood-space (p. 362). In the remainder of the body the 

 space between the epiderm and the viscera is filled by 

 the muscles and connective tissue. 



The 7nouth (Fig. 93, mth) lies in the middle line just 

 below the anterior adductor. On each side of it are two 

 triangular flaps, the internal and external labial palps ; the 

 external palps unite with one another in front of the mouth, 

 forming an upper lip; the internal are similarly united 

 behind the mouth, forming a lower lip : both are ciliated 

 externally. The mouth leads by a short gullet (gut) into a 

 large stotnach (si), which receives the ducts of a pair of 

 irregular, dark-brown, digestive glandiXd. gl). The intestine 

 (int), which is lined by a ciliated epithelium, goes off from 

 the posterior end of the stomach, descends into the visceral 

 mass, where it is coiled upon itself, then ascends parallel to 

 its first portion, turns sharply backwards, and proceeds, as 

 the rectutn {ret), through the pericardium where it traverses 

 the ventricle of the heart, and above the posterior adducton 

 finally discharging by the anus {a) into the exhalant siphon, 

 or cloaca. The wall of the rectum is produced into a 

 longitudinal ridge, or typhlosole (Figs. 93 and 95, ty), and 

 two similar ridges begin in the stomach and are continued 

 into the first portion of the intestine. The stomach con- 

 tains at certain seasons of the year a gelatinous rod, the 

 crystalline style. 



The gills consist, as we have seen, of two plate-like 



