394 THE MUSSEL chap. 



gills, and posterior portion of the mantle. The nerve-cells 

 are confined to the ganglia. 



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 the 

 Earthworm and Crayfish, and the pedal ganglia as an infra- 

 cesophageal mass representing the ventral nerve-cord. 



Sensory organs are poorly developed, as might be ex- 

 pected in an animal of such sedentary habits. In connec- 

 tion with each visceral ganglion is a patch of sensory 

 epithelium forming the so called olfactory organ or, better, 

 ospJiradium, the function of which is apparently to test the 

 purity of the water entering by the respiratory current. Close 

 to the pedal ganglion a minute " auditory sac " or otocyst is 

 found, the nerve of which is said to spring from the cerebro- 

 pedal connective, being probably derived from the cerebral 

 ganglion. Sensory cells, probably tactile, also occur round 

 the edge of the mantle, and especially on the tentacles of 

 the inhalant siphon. 



The sexes are separate. The gonads (Fig. 93 gon) 

 are large, paired, racemose (p. 135) bodies, occupying a con- 

 siderable portion of the visceral mass amongst the coils of 

 the intestine : the spermary is white, the ovary reddish. The 

 gonad of each side has a short duct which opens (^.c/) on 

 the surface of the visceral mass, just in front of the renal 

 aperture. 



In the breeding season the eggs, extruded from the genital 

 aperture, pass into the supra-branchial chamber, and so to 

 the cloaca. There, in all probability, they are impregnated 

 by sperms introduced with the respiratory current. The 

 oosperms are then passed into the cavities of the outer gills, 



