68 



THE BOOK OF NATURE STUDY 



is ended, the establishment of two adductor muscles, of the ali- 

 mentary canal, of the beginnings of the gills and other organs, 

 and eventually of the permanent shell. The last is formed under- 

 neath the glochidial shell, and is at first quite transparent. These 

 changes accomplished, the animal drops off the fish, and begins an 

 independent life as a very small mussel. 



Thus Anodonta secures a wide dispersal, despite the sacrifice 

 of the free-swimming, ciliated (veliger) stage, by calling to 



its aid an active, strong- 

 swimming creature that is 

 well able to make headway 

 against natural currents of 

 water and avoid being carried 

 out to sea. It is not known 

 whether glochidia can sur- 

 vive immersion in salt water 

 when attached to fishes which 

 go down to and return again 

 from the sea. When attached 

 to tadpoles they usually, per- 

 haps always, perish. 



In Central Europe, but 

 not in these islands, there is 

 a curious exchange of compli- 

 ments between the mussels 

 and a small carp-like fish, 

 the "bitterling" (Rhodes 



amarus\ to which the glochidia there become fastened. The 

 female bitterling has at the spawning season a very long ovi- 

 positor, which she insinuates between the gaping valves of the 

 mussel shell into the gill-cavities and there deposits her eggs. 

 The young bitterlings undergo their early development under the 

 protection of the mussel, though apparently not at her expense 

 in any way, and leave their host when they have attained a length 

 of about i cm. 



Oysters, cockles, edible (marine) mussels, clams, and all 

 bivalves belong to the same division of molluscs as do the fresh- 

 water mussels. The division is known as the Lamellibranchs 



FIG. 38. Ventral view of young mussel shortly 

 after the commencement of free life, a, pos- 

 terior adductor muscle ; b, anterior adductor 

 muscle ; c, gill filaments ; d, permanent 

 shell ; ^, teeth of glochidial shell ; foot. 



