190 Shell Life 



the, in some places considerable, percolations from 

 the cliffs, so also the shape of the shell appears to 

 have been modified. Canon Norman pointed out 

 years ago that "the nearer high-water mark tlie 

 shell is taken, the higher-spired, more strongly ribbed, 

 and smaller it will be; a-nd that the lower down it 

 lives, the flatter, less ribbed, and larger it becomes." 

 This result, I should suppose, is due to the action of 

 natural selection. One of the principal enemies of 

 the Limpet is the Oyster-catcher, whose bill is admir- 

 ably adapted for wrenching it from the rock when 

 its shell is slightly lifted for fresh air or locomotion. 

 It will be evident that the flatter, smoother, and 

 larger the shell may be, the more easily it may be 

 lifted, so that the birds are more likely to select such 

 specimens in preference to the less easily obtained 

 small, high-spired, and ridged examples : the former, 

 therefore, must become scarce on the rocks most 

 quartered by the larger shore -birds. 



The tongue {radula) of the Limpet when uncoiled 

 is found to exceed the shell in length ; it is 

 furnished with nearly 2000 teeth, with which it 

 scrapes the surface of the rocks, clearing off the 

 delicate growths of weeds and leaving a peculiar 

 zioi:zao; track. 



The means by which the Limpet adheres to the 

 rock has long been a fruitful subject for controversy, 

 many naturalists appearing unwilling to believe that 

 it could be held there by the same powers as those 

 that hold the Winkle — the muscles and mucous of 

 the foot, which by excluding air establish so close a 

 contact that considerable pulling force is required to 

 dislodge it. Eeaumur experimented with the Limpet's 



