CIRCULATORY SYSTEM 



407 



The circulatory system is well developed. The heart lies 

 in the pericardium, and consists of a single ventricle (Figs. 

 101 and 103 B, v) and of right and left auricles (au). The 

 ventricle is a muscular chamber which has the peculiarity 

 of surrounding the rectum (ret] : the auricles are thin-walled 



FIG. 104. Diagram of the circulatory system of A nodonta. 



The blood received from the auricles (au) is pumped by the ventricle (v) into the 

 aorta (ad) and thence passes to the mantle (arfi.) and to the body generally 

 (arft.). The blood which has circulated through the mantle is returned directly 

 to the auricle : that from the body generally is collected into the vena cava (v. c), 

 passes by nephridial veins (nph. v) to the kidneys, thence by afferent branchial 

 veins (of. br. v) to the gills, and is returned by efferent branchial veins (ef. br. v) 

 to the auricles ; pc. pericardium. (From Parker and Haswell's Zoology,) 



chambers communicating with the ventricle by valvular 

 apertures opening towards the latter. From each end of the 

 ventricle an artery is given off, the anterior aorta (Fig. 101, 

 a. ao] passing above, the posterior aorta (p. ao] below the 

 rectum, From the aortae the blood passes into arteries (Fig. 



