Tusk-shells, Mail-shells, and Limpets 191 



power in this respect, and says tliat for several 

 seconds it supported a quarter of a hundredweiglit 

 attached to its shell. Some authors imagined that the 

 foot contained a number of— presumably invisible- 

 suckers ; others, that by drawing up the centre, after 

 the manner of the street-boy's leathern sucker, it 

 created a vacuum which held it to the rock. Reaumur 

 was troubled by some of these notions, but he disposed 

 of the vacuum theory by cutting a Limpet — shell and 

 all_right through, and as the two halves still 

 adhered to the rock it was clear there could be no 

 vacuum. 



It has long been noticed that the Limpet sinks 

 pits corresponding exactly to the margin of the shell, 

 but deep or shallow according to the softness or 

 hardness of the rock. In this way it overcom.es the 

 difficulty of shutting down its shell closely upon an 

 irregular surface, by making its own little patch 

 level. Various agencies have been invoked to explain 

 how these pits are sunk; an acid secreted by the 

 foot, and carbonic acid given off in respiration, being 

 among them. No doubt on chalk and other soft 

 rocks we need look no further than to the pulling 

 action of muscular contractions of the foot. Even 

 on the hard slaty rocks of the Cornish coast, I have 

 frequently noticed that Limpets taken off for use as 

 bait have thin laminse of rock still attached to the 

 foot. But the most remarkable of the Limpet's 

 habits is that of returning to the identical spot to 

 roost after each excursion in search of food. Given 

 a particular area of rocks it would be almost impossible 

 to find two pits so exactly alike that they would 

 equally serve as lodgment for a particular Limpet. 

 13 



