.„■ 200 



Microns 



Figure 212. — Transverse section of a portion of the peri- 

 cardium wall of C. virginica with an artery surrounded 

 by large vesicular cells, nematopsis cysts on upper right 

 and lower left sides. Bouin, hemato.xylin-eosin. 



framework of muscle fibers and connective tissue 

 cells forming an irregular trabecular structure 

 (fig. 216), with amoebocytes in the spaces between 

 the fibers and in the connective tissue. The outer 

 surface of the ventricle is covered with epithelium 

 of a single layer of flat and thin cells with con- 

 spicuous nuclei. 



The walls of the auricles, thinner and lighter than 

 those of the ventricle, also form a trabecular 

 framework supported by connective tissue (fig. 

 217). 'Amoebocytes are numerous between the 

 connective tissue cells and along the muscle fibers. 

 On the outside the auricles are covered with tall 

 colunmar epithelium which contains many glan- 

 dular and dark pigment cells; this epithelium 

 constitutes a part of the excretory system in 

 bivalves (Franc, 1960, p. 2016). Neither the ven- 

 tricle nor the auricles has an inner epithelial 

 lining. 



The movement of blood from the auricles to 

 the ventricle is controlled bj' the two auriculo- 

 ventricular valves which appear as circular bands 

 of tissue surrounding small openings (fig. 218). 

 In longitudinal section the auriculo-ventricular 

 valve (fig. 219) resembles a convoluted cylindrical 

 tube. The walls of the valves consist of several 

 layers of muscle fibers arranged obliquely and 

 supported by connective tissue. When the auricle 

 (left part of fig. 219) contracts, blood is propelled 

 into the ventricle (right portion of the figure), 

 which in turn contracts, compressing the walls 

 of the valves and forcing the blood forward into 

 the aorta (not shown in fig. 219). 



The heart is well supplied with ganglion cells 

 and nerve fibers which end in the muscles. Prep- 

 arations of heart tissue of C. virginica stained 

 with methylene blue and examined in glycerin 

 under oil immersion showed a great abimdance of 

 these elements (fig. 220). These observations 

 support the findings of Suzuki (1934a, 1934b), 

 who described the ganglion cells in the hearts of 

 Ostrea circumpicta Pils., 0. ijifias Thunb., and 

 Pinctada martensi. According to his data, the 

 ganglion cells in these oysters are particularly 

 abundant at the septum separating the auricles 

 from the ventricle where they form a ring at 

 the narrowest portion of the heart. Direct con- 



Microns 



Figure 213. — Transverse section of the pericardium wall 



, of C. virginica with the branch of the cardial nerve (cut 



at a slightly slanted angle). Bouin, hematoxylin-eosin. 



CIRCULATORY SYSTEM AND BLOOD 



241 



