642 



ZOOLOGY 



SECT. 



ot 



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 Annelids and Arthropods, and the 

 pedal ganglia as an infra-oesophageal 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, osghradium, 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 otocyst 

 (Fig. 532) is sometimes found, 

 the nerve of which is said to 

 spring from the cerebro-pedal 

 connective, being probably 

 derived from the cerebral 

 ganglion. Sensory cells prob- 

 ably tactile also occur round 

 the edge of the mantle, and 

 especially on the fimbria? of 

 the inhalant siphon. 

 Reproductive organs. The sexes are separate. The gonads 

 (Fig. 528, gon.} are large, paired, racemose glands, occupying a 

 considerable portion of the visceral mass amongst the coils of 

 the intestine : the testis is white, the ovary reddish. The gonad 

 of each side 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 suprabranchial 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, which they distend 

 enormously. Thus the outer gills act as brood-pouches, and 

 in them the embryo develops into the peculiar larval form 

 presently to be described. 



FIG. 532. Otocyst of Anodonta. a, I, c, c', 

 cellular layers surrounding the otocyst ; ot. 

 otolith. (From the Cambridge Natural 

 History.) 



