



Microns 



200 



Figure '211. — Transverse section of the pericardium wall of C. virginica. Surface epithelium is rich in mucous (light) and 

 eosinophilic cells (dark granules). Large vein (right) and blood sinus (left). The epithelium of tlie inner sides 

 (lower side of the drawing) faces the heart. Bouin, Mallory triple stain. 



facing the heart consists of small flattened cells 

 and a few scattered eosinophilic and mucous cells; 

 on the opposite side, facing the shell, the peri- 

 cardium wall is covered with large columnar 

 epithelial cells with oval nuclei and many eosino- 

 philic and mucous cells. Basal membrane on the 

 upper side of the wall is well developed. 



THE HEART 



The three-chambered heart is suspended 

 obliquely in the pericardium and is held by the 

 root of the aorta on one side and by tlie common 

 efferent veins on tlie other. The ventricle is 

 larger and bulkier than the two auricles; a con- 

 striction between the ventricle and auricles marks 

 the partitition between them (fig. 214). The 

 auricles are darkened by pigment cells in tlieii' 

 walls. The degree of pigmentation varies from 



light brown to almost black. The ventricle is a 

 pear-shaped structure slightly constricted along 

 tlie middle. Its walls are formed by tliick bundles 

 of nonstriated muscle fibers which traverse the 

 ventricular cavity and incompletely divide it into 

 two chambers. 



In the majority of bivalves the rectum passes 

 tlu-ough the lieart, but in the oyster the rectum 

 lies behind the heart (fig. 71). 



The fibers of the heart muscle cross one another 

 in many directions, fref}uently branch and anas- 

 tomose, and are surrounded by delicate connec- 

 tive tissue. In general the muscle tissue has a 

 spongy appearance (fig. 215). In the ventricle 

 the nmscle fibers are thicker and stronger than in 

 the auricles. 



The wall of tlie ventricle and the septum be- 

 tween the two parts of the heart are formed by a 



240 



FISH AND WILDLIFE SERVICE 



