PHYLUM MOLLUSCA 



691 



ol 



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 in part with the brain of Annelids and Arthropods, 

 and the pedal ganglia as an infra-cesophageal 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 

 ganglion a minute statocyst 

 (" otocyst ") (Fig. 570) is some- 

 times found, the nerve of which 

 is said to spring from the 

 cerebro-pedal connective, being 

 probably derived from the cere- 

 bral ganglion. Sensory cells 

 probably tactile also occur 

 round the edge of the mantle, and especially on the fimbriae of 

 the inhalant siphon. 



Reproductive organs. The sexes are separate. The gonads 

 (Fig. 566, 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 gill-laminae, which they 

 distend enormously. Thus the outer gill-laminae act as brood- 

 pouches, and in them the embryo develops into the peculiar larval 

 form presently to be described. 



FIG. 570. Statocyst of Anodonta. a,b,c,c', 

 cellular layers surrounding the statocyst ; ot. 

 statolith. (From the Cambridge. Natural 

 History.) 



