276 MANUAL OF ZOOLOGY SECT. 



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. 



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 otocyst is some- 

 times found. Sensory cells probably tactile also occur 

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

 of the inhalant siphon. 



The sexes are separate. Ite gonads (Fig. 150, 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 a peculiar larval form known as glochidium. 



The glochidium (Fig 153) has a bi-valved shell produced 

 ventfally into incurved hooks beset with spines. After a 

 time it is ejected from the mantle-cavity and falls to the 

 bottom of the water, where it lies until it has the oppor- 



