CH. VI] 



THE FRESH-WATER MUSSEL 



195 



dorsal surface. This notch (x, Fig. 37) persists through 

 life and causes the slight dorsal of the curves marking the 

 lines of growth. In each successive line of growth the 

 notch becomes of greater antero-posterior and less dorso- 

 ventral extent, thus tending to become less evident and to 

 disappear. It can therefore be most easily seen near the 

 umbones of those shells which have escaped corrosion. 



The foot of the young mussel is blunt and far flatter 

 than that of the adult. When the animal is advancing, 

 the foot at first protrudes slowly until it stretches a con- 



B 



v.m. 



Fig. 38. Young mussel about seven weeks after close of parasitic life 

 and 129 days after first attachment to stickleback, with glochidial 

 shell as a saddle over the delicate permanent shell. In A the animal 

 is slowly pushing the ciliated foot forward in the direction indicated 

 by the arrow. In B the foot is fixed, the cilia motionless and the 

 shell is being drawn forward, its valves are drawn together by the 

 adductors so as to pinch the hinder part of the foot. 



/. Foot. g.f. Three first gill filaments seen through shells. 



g.s. Glochidial shell. p.s. Permanent shell. v.m. Visceral 



mass. Magnified. 



132 



