VTII 



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 



arl.i- 



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. ad] extending 

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

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



D D * 



