392 THE MUSSEL chap. 



The circulatory system is well developed. The heart lies 

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

 93 and 95 B, v) and of right and left auricles {aii). The 

 ventricle is a muscular chamber which has the peculiarity 

 of surrounding the rectum {ret) : the auricles 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 93, 

 a. ao) passing above, the posterior aorta {p. ao) below the 

 rectum. From the aortse the blood passes into arteries (Fig. 

 96, art.} art') which ramify all over the body, finally forming 

 an extensive network of vessels, many of which are devoid 

 of proper walls and have therefore the nature of sinuses. 

 The returning blood passes into a large longitudinal vein, 

 the vena cava (Figs. 95 and 96, v. c), placed between the 

 nephridia, whence it is taken to the kidneys themselves 

 (Fig. 96, npli. v), thence by afferent branchial veins (af. br. v) 

 to the gills, and is finally returned by efferent branchial veins 

 (ef. br. v) to the auricles. The mantle has a very extensive 

 blood supply, and probably acts as the chief respiratory organ 

 (p. 390) . its blood {art^) is returned directly to the auricles 

 without passing through either the kidneys or the gills. 

 The blood is colourless and contains leucocytes. There is 

 no communication between the blood-system and the 

 pericardium. 



The nervous system is formed on a type quite different 

 from anything we have yet met with. On each side of the 

 gullet is a small cerebropleural ganglion (Fig. 93, c. pi. gn), 

 united with its fellow of the opposite side by a nerve-cord, 

 the cerebral commissure, passing above the gullet. Each 

 cercbro-pleural ganglion also gi\es off a cord, the cerebro- 

 pedal connective, which passes downwards and backwards to 

 a pedal ganglion (pd. gn) situated at the junction of the 



