362 THE FRESH-WATER MUSSEL less. 



respiratory organ : 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 cerebro-pleural ganglion (Fig. 94, 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. PVom 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 {v. gn) placed on 

 the ventral side of the posterior adductor muscle. The 

 visceral, like the pedal ganglia, are fused together. 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, gills, and 

 posterior portion of the mantle. 



It will be seen that the cerebral commissures and cerebro- 

 pedal connectives, together with the cerebro-pleural and 

 pedal ganglia, form a nerve-ring which surrounds the gullet : 

 the cerebro-pleural ganglia may be looked upon as a supra- 

 cesophageal nerve mass corresponding with the brain 

 of Polygordius and the Crayfish, and the pedal ganglia 

 as an infra-oesophageal mass representing the ventral nerve 

 cord. 



Sensory organs are poorly developed, as might be ex- 



