xxviii DEVELOPMENT 363 



pected in an animal of such sedentary habits. In connec- 

 tion 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 is a minute otocyst or statocyst, 

 the nerve of which is said to spring from the cerebro-pedal 

 connective, being probably derived from the cerebral gang- 

 lion. Sensory cells, probably tactile, also occur round the 

 edge of the mantle, and especially on the fimbrise of the in- 

 halant siphon. 



The sexes are separate. The gonads (Figs. 93 and 94, 

 gon) are large, paired, racemose bodies, occupying a con- 

 siderable portion of the visceral mass amongst the coils of 

 the intestine : the spermary 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 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 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. 



The segmentation of the oosperm is remarkable for the 

 fact that the cells of the polyplast are of two sizes, small 

 cells composed entirely of protoplasm, and large cells loaded 

 with yolk -granules. In the formation of the gastrula the 

 large are invaginated into the small cells, but the enteron 

 thus formed is very small and quite unimportant during 

 early larval life, the young mussels being nourished, after 



