408 THE MUSSEL CHAP. 



104, art l ,arfi) 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. 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 

 (ef. 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 

 pericardium. 



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.pl. 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 enteric canal, heart, 



