408 THE MUSSEL CHAP. 



art*, art 2 } 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 longi- 

 tudinal vein, the vena cava (Figs. 103 and 104, v. c), 

 placed between the nephridia, whence it is taken to the 

 kidneys themselves (Fig. 104, nph. v), thence by afferent 

 branchial veins (af. br. v) to the gills, and finally returned 

 by efferent branchial veins (e. br. v) to the auricles. The 

 mantle has a very extensive blood-supply, and probably 

 acts as the chief respiratory organ (p. 406) : its blood 

 (art 1 ) 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 peri- 

 cardium. 



The nervous system is formed on a type quite different 

 from anything we have yet met with. On either side of 

 the gullet is a small cerebro-pleural ganglion (Fig. 101, 

 c. pi. gn), united with its fellow of the opposite side by a 

 nerve-cord, the cerebral commissure, passing above the 

 gullet. Each cerebro-pleural ganglion also gives off a 

 cord, the cerebro-pedal connective, which passes downwards 

 and backwards to a pedal ganglion (pd. gn) situated at 

 the junction of the visceral mass with the foot ; the two 

 pedal ganglia are so closely united as to form a single 

 bilobed mass. From each cerebro-pleural ganglion 

 there further proceeds a long cerebro-visceral connective, 

 which passes directly backwards through the kidney 

 and ends in a visceral ganglion (Figs. 101 and 103 C, v. gn) 

 situated on the ventral side of the posterior adductor 

 muscle. The visceral, like the pedal ganglia, are fused 

 with one another. The cerebro pleural ganglia supply 

 the labial palps and the anterior part of the mantle ; 

 the pedal, the foot and its muscles ; the visceral, the 



