20 



LIMPET. 



determined ; but now we submit the shell to the 

 searching powers of the microscope, and find 

 that various kinds of shells are characterised 

 by various arrangements of particles, and are 

 acted upon by polarized light in various ways. 

 It is, therefore, quite possible to fix the character 

 of a shell from a single fragment no larger than 

 a pin's head. There are few things more curious 

 than this wonderful arrangement of the particles ; 

 which, by the way, are brought within the scope 

 of the microscope, by making very thin sections 

 of the shells, by the aid of saw, file, and hone. 



One of the commonest shells found on the 

 sea-shore is the Limpet {Patella vulgata). See 

 plate B, fig. 3. 



In its living state it may be found adhering 

 closely to rocks or other substances, that give it 

 a firm basis of support. The adhesion is caused 

 by atmospheric pressure, for the limpet is enabled 

 to raise the centre of that part of the body that 

 rests on the rock, while the edge is closely pressed 

 upon it. This movement causes a vacuum ; and 

 so firmly does the air hold the limpet in its 

 place, that the unaided fingers will find great 

 difficulty even in stirring it. The firmness of 

 adhesion is also increased by the fact that after 

 the animal has remained for some time in one 



