ACTION OP LIMPETS ON CHALK. 409 



limpets, remind one, in a small way, of the llanos Oi", pampas on 

 the land, where arboreal vegetation is kept down by herbivorous 

 animals. Yet the limpets appear to do their work more eifeetu- 

 ally, as they uproot all alien growths. 



The holes in the chalk, in which the limpets are often to be 

 found, are, I believe, excavated in a great measure by rasping 

 with the lingual teeth, though I doubt whether the object is to 

 form a cavity to shelter in, though the cavities, when formed, may 

 be of use for that purpose. It must be of the greatest importance 

 to a limpet that, in order that it may ensure a firm adherence to 

 .the rock, its shell should fit the rock accurately ; when the shell 

 does fit the rock accurately, a small amount of muscular contrac- 

 tion of the animal would cause the shell to adhere so firmly to a 

 smooth surface as to be practically immovable without fracture. 

 As the shells cannot be adapted daily to difierent forms of 

 surface, the limpets generally return to the same places of at- 

 tachment. I am sure this is the case with many ; for I found 

 shells perfectly adjusted to the uneven surfaces of flints, the 

 growth of the shells being in some parts distorted and indented 

 to suit inequalities in the surface of the flints. As the edges of 

 the shells, especially those of the younger animals, are very sharp, 

 the eff"ect of pressure brought to bear on the edge, either by the 

 contraction of the animal or by the shock of the waves, would, if 

 there is the least sideway movement, be to cut into the chalk 

 round the edge of the shell. The muscles of the animal are ge- 

 nerally relaxed when reposing ; for if the point of a knife be 

 quickly inserted beneath the edge of the shell, it may be detach.ed 

 from the rock without difficulty; but if the least warning by a 

 touch be given to the animal, its muscles contract, and it adheres 

 so firmly that it is impossible to detach it without breaking the 

 edge of the shell *. These alternate relaxations and contractions 

 on sudden alarms would tend to increase the effect of the cutting 

 action of the edge of the shell. I saw the fine indentations 

 round the edge of some of the shells exactly reproduced upon the 

 surface of the chalk ; and this could only result from pressure on 

 the shell forcing its sharp edge into the chalk. A very little 

 pressure, as may be found by trial, will sufiice to force the edge 

 of the shell into the chalk. The effect of the formation of a 

 groove in the chalk corresponding with the edge of the shell 



* Reaumur found that a limpet could sustain a weight of from 28 to 30 . 

 pounds for some seconds (Jeffreys, 'Brit. Oonch.' toI. iii. p. 232). 



