xi PHYLUM MOLLUSCA 277 



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 form- 

 ing 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 sometimes found. Sen- 

 sory cells — probably tactile — also occur round the edge of 

 the mantle, and especially on the fimbriae of the inhalant 

 siphon. 



The sexes are separate. The gonads (Fig. 162, gon) are 

 large, paired, racemose glands, occupying a considerable 

 portion of the visceral mass amongst the coils of the intes- 

 tine ; the testis is white, the ovary reddish. The gonad of 

 each side has a short duct which opens (g. ap) on the sur- 

 face of the visceral mass just in front of the renal aperture. 



In the breeding season the eggs, extruded from the gen- 

 ital 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. 165) has a bivalved shell produced 

 ventrally 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- 

 tunity of becoming attached to the gills or skin of a fish. 

 Fixed firmly by means of the hooked valves the larva remains 

 as an external parasite for about ten weeks, becoming en- 



