viii SENSORY AND REPRODUCTIVE ORGANS 409 



enteric canal, heart, 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 supraoesophageal nerve-mass corre- 

 sponding with the brain of the earthworm and crayfish, 

 and the pedal ganglia as an infraoesophageal mass 

 representing the ventral nerve-cord. 



Sensory organs are poorly developed, as might be 

 expected in an animal of such sedentary habits. In 

 connection with each visceral ganglion is a patch of 

 sensory epithelium forming the so-called " olfactory 

 organ," or, better, osphradium, the function of which is 

 apparently to 'test the purity of the water entering by 

 the respiratory current. Close to the pedal ganglia a 

 minute paired stalocyst, containing a calcareous lithite 

 (pp. 315 and 381), 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. 101, gon) 

 are large, paired, racemose (p. 133) bodies, occupying a 

 considerable portion of the visceral mass amongst the 

 coils of the intestine : the spermary is white, the ovary 

 reddish. Each gonad has a short duct which opens 

 (g. ap) 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 



D D 2 



