Chap. XVI.] THE FRESH-WATER MUSSEL. 321 



swim by flapping the valves. Ere long they become attached 

 by .the byssus-thread to a fish, and hang there snapping their 

 valves until they bury them in the epidermis of the fish. There 

 they become encysted by a morbid growth of the epiderm cells 

 of their host ; and there they undergo a post-embryonic meta- 

 morphosis. The byssus and its secreting organ atrophy, and the 

 tactile organs disappear. The single adductor also atrophies; 

 but ^before it has disappeared rudiments of the anterior and 

 posterior adductors of the adult become visible. The foot arises 

 from the point at which the byssus disappeared. The gills arise 

 as papillae covered with richly ciliated epidermis. The mantle- 

 lobes undergo great change, or are perhaps formed de novo. The 

 permanent shell is formed as two plates on the surface of the 

 still parasitic larva. Finally, the little anodon, now resembling 

 in all essential features the adult, is set free and dropped to 

 the bottom. 



The glochidium embryo is peculiar to fresh-water mussels 

 (anodons and unios). The great majority of lamellibranch 

 molluscs are hatched as free-swimming embryos with a ciliated 

 velum. It is clear, however, that such free-swimming embryos 

 of fresh-water mussels, did they exist, would be liable to be 

 swept out to sea by the flow of the river water in which they 

 lived. On the other hand, if they were developed under the 

 protection of the parent until they acquired the adult condition 

 and sedentary habit, there would be little dispersal of the genus. 

 The attachment of the glochidium to the epidermis of the fish, 

 while it (1) prevents the embryos being swept away by the 

 seaward current of the river, at the same time (2) provides an 

 efficient means of dispersal. 



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