42 



Our Food Mollusks 



slightly different shape in different bivalves, as indicated 



in the figure. 



After becoming mature in the sexual glands of bi- 

 valves, usually in the early sumrher, both male and fe- 

 male cells are discharged into the water. Both may be 



Fig. io. — Ovarian ovum of the oyster (o) 

 showing nucleus (n) and nucleolus (n'). 

 The relative size of the male cell is repre- 

 sented by the small figure lying near it. 

 There are also shown, greatly magnified, the 

 spermatozoa of Ostrea (a), Pecten (b), 

 and Venus (c). 



carried by currents. The ova are passive and gradually 

 sink, but the spermatozoa very soon begin to swim. 

 They are vastly more numerous than the eggs which they 

 are intended to find. Though a single spermatozoon 

 only, unites with an ovum in normal cases, it might ap- 

 pear that conditions are such that comparatively few 

 unions would take place, and the opinion is sometimes 

 expressed that such probably is the case. There is, how- 

 ever, no good reason for such a belief. Males and fe- 

 males with full glands, may be stimulated experiment- 

 ally by employing certain changes in the surroundings 



