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THE BOOK OF NATURE STUDY 



Anodonta. The eggs of the female pass from the genital opening 

 backward to the exhalant siphon, but, instead of leaving the 

 shell, they then are driven forward into the cavities of the ex- 

 ternal gills. This forward movement is probably achieved by 

 a sort of gulp effected by the sudden relaxation of the adductor 

 muscles, while the whole gape of the valves is blocked by the 

 mantle, except at the siphons, where there consequently results 

 a violent inrush of water. The eggs are fertilised while in the 

 external gills, the sperms from the males being conveyed thither 

 by the currents of water, though it should be stated that at the 

 spawning season the animals congregate in the shallower water. 

 The production of eggs continues for about ten days, and during 

 that time about half a million eggs pass to the external gills. 

 There they undergo their development within their egg-shells, 

 nourished at first upon the yolk contained, but subsequently 

 by a nutritive slime discharged by the gill itself. At one period 

 the embryo is furnished with cilia, and slowly swims round and 

 round within the egg-shell. This ciliated stage (veliger) is a 

 heritage from the days when the ancestors of Anodonta lived in 



the sea. Many marine 

 molluscs emerge from the 

 egg - shell at this stage 

 and, swimming in the 

 open sea, become spread 

 out over far wider areas 

 than could be the case if 

 their powers of dispersal 

 were confined to the 

 adult stage, which is 

 often stationary. But 

 now that Anodonta lives 

 in fresh water, freedom 

 of the ciliated young 

 would be fraught with the peril of being washed down stream 

 and eventually into salt water, which would be fatal. Hence 

 we find, as so often in freshwater animals, that the young 

 are retained within the shelter of the parent until a far 

 later period, and gain dispersal by other less risky methods. 



FIG. 36. Ventral view of glochidium. <z, adductor 

 muscle showing through mantle ; , byssus. 



