SHELLS AND MOLLUSCOUS ANIMALS. 213 



stone, the base of the shell is flattened, and the 

 mouth roundish ; but if it grows attached to some 

 crag, or on the cavity of some refuse shell, the base 

 is flat and internally convex, and as the limpets 

 move from place to place, several changes may 

 occur in the form of the same individual shell. 

 Some species seem to move continually from one 

 reef or rock to another, but others are fixed, and 

 appear to depend for sustenance on such food as 

 the wave may bring them. Yet it is generally 

 believed that they can eat marine plants, and the 

 long and curiously formed tongue of our common 

 limpet, armed with spines and tiny hooks, is a 

 wondrous piece of mechanism, admirably adapted 

 for sawing the sea-weeds into little pieces. 



The Patellse are found on rocky coasts, on 

 almost all shores, excepting those of the Arctic 

 seas ; and as they grow to a much larger size on 

 tropical rocks, they form a most valuable article 

 of food. Even our smaller native species are of 

 great value in some parts of this kingdom, and 

 the peasantry of the Western Isles of Scotland 

 look to the periwinkles and limpets which abound 

 on their rocks for their daily meal, often, for long 

 seasons, subsisting almost entirely upon this 

 humble food. In the Isle of Skye the inhabitants 

 are often, at one time of the year, without any 

 other source of provision. Darwin, in his Journal 

 of Researches, describes the poor people of Tierra 

 del Fuego as living almost entirely on the mol- 

 luscous animals which they can gather from their 

 rocks. " Whenever," says this writer, " it is low 

 water, winter or summer, night or day, they must 

 rise to pick up shell-fish from the rocks ; and the 

 women either dive to collect sea eggs, or sit 



