1 8 THE STORY OF ANIMAL LIFE 



shells adapt their forms to their surroundings. 

 In each case the shell is a plain conical cap, and 

 the animal within keeps the shell firmly attached 

 to the base on which it rests. The Limpet can 

 move about at a very creditable snail's pace 

 when it wishes to do so, and at low-water mark, 

 when the tide is beginning to rise, you may easily 

 find them moving about and off their guard; but 

 during many hours of the day, when the tide is 

 out, the main object of the Limpet is to keep its 

 shell as firmly fixed to the rock as possible. It 

 will at once be seen that if the margin of the 

 shell were smooth like that of a tea-cup, and the 

 surface of the rock to which it clung very irregu- 

 lar, many chinks would be left between the mar- 

 gin of the shell and the surface of the rock 

 through which unwelcome visitors might find en- 

 trance. The loss of moisture through the crev- 

 ices, too, would be a serious thing to the animal 

 during the hours when the shell is uncovered by 

 the tide and exposed to the rays of a hot sun. 

 On the other hand, if the margin of the shell 

 were irregular, and the surface on which it rested 

 smooth, unprotected crevices would in the same 

 way be left. So the Limpets adapt the shape of 

 their shell to their surroundings; \.\\e Patella pellu- 

 cida, which lives on the smooth branches of the tan- 

 gle-root, has a shell with a smooth regular edge; 

 while the Patella vulgata, which lives upon rocks, 

 has a shell with an irregular, indented edge, 

 whose irregularities fit into those of the rock on 

 which it rests. (See Fig. 2.) 



Probably every reader will be able to appre- 

 ciate the above instances of creatures adapted to 

 their surroundings. For there are few people 

 who are not familiar with the common Limpet of 



