156 THE HARMONIES OF NATURE. 



trance of their house, and closes it like a fortress against the 

 outer world. But no animal exists that is safe against every 

 attack, for the large sea-birds sometimes carry the ponderous 

 snails, whose entrance they cannot force with their beaks, high 

 up into the air, and let them fall upon the rocks, where they 

 are dashed to pieces. 



The limpets, slowly crawling over the stones under their 

 shield-like cover, have no operculum to close its entrance, but 

 their broad-soled foot renders a door perfectly unnecessary ; 

 for, acting like a powerful sucker, it clings with such tenacity to 

 the rock, that it requires the introduction of a knife between 

 the shell and the stone to detach them. It has been calculated 

 that the larger species are thus able to produce a resistance 

 equivalent to a weight of 150 Ibs., which, considering the sharp 

 angle of the shell, is more than sufficient to defy the strength 

 of a man to raise them. It is also said that crows, and other 

 birds, which endeavour to detach them for food, are sometimes 

 caught by the points of their bills, and are held there until 

 drowned by the advancing tide. 



The land-snails have also no operculum, but before they fall 

 into their winter-sleep, they close the mouth of their shells 

 with a calcareous secretion which, stopping it up, entirely 

 protects it from every external injury. In the centre is an 

 extremely minute orifice, communicating with the lungs ; and 

 this minute hole, though not large enough to admit a drop of 

 water, is of sufficient capacity for the passage of air. Not 

 unfrequently, on removing this cover, a second or even a third 

 similar one will be found within, forming additional safeguards 

 against intrusion or the vicissitudes of temperature. 



When the genial warmth of spring awakens the snail to a 

 state of activity, its covering, now no longer needed, drops off, 

 and the animal, protruding its horned head, sets out in quest of 

 food, anxious to make amends for its long abstinence by 

 feasting on the first tender leaves that fall in its way. 



The respiratory organs of the gasteropoda are found to be 

 constructed upon very various principles, according to the 

 medium which they inhabit or the peculiarities of their mode of 

 life. The common land -snails, as well as the freshwater snails, 

 breathe air, which is received into a cavity lined with delicate 

 network, analogous to the lungs of air-breathing animals, and 



