194 



THE FRESH-WATER MUSSEL 



[CH. VI 



way along its length. Hence at this point the permanent 

 shell is prevented from growing so fast as elsewhere and 

 therefore has its otherwise symmetrical curve sharply 

 interrupted by an irregular notch pointing towards the 



Fig. 36. A ventral and a lateral view of shells of young mussel at close 

 of parasitic life, to show relation of permanent shell to glochidial 

 shell. Magnified. 



Fig. 37. A lateral and a dorsal view of a valve of shell of Unio, to show 

 effect of glochidial shell-teeth upon lines of growth of mature shell. 

 Reduced. 



