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 



art./ 



FIG. 104. Diagram of the circulatory system of Anodonta. 



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

 aorta (ao) and thence passes to the mantle (arfl) and to the body generally 

 (art *). 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 (af. br. v) to the gills, and is returned by efferent branchial veins (ef. br. r) 

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



are thin- walled 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) extending 

 above, the posterior aorta (p. ao) below the rectum. 

 From the aortae the blood passes into arteries (Fig. 104, 



D D * 



