144 FORMATION OP SHELL. 



may be seen just protruded round the aperture, as the 

 creature crawls along; but in the Cyprcea, or Cowry, 

 and in such shells as have a similarly polished coat, the 

 mantle folds back over the surface of the shell, to which 

 it imparts the high polish and the beautiful markings 

 these shells display. The annexed 

 section of the shell otNassa reticulata 

 is intended to show the nature of 

 its internal spires. 



Notwithstanding the defences pro- 

 vided by Nature for the shelly Mol- 

 luscs, they have many enemies, from 

 whose attacks the largest and strongest 

 shells do not always afford protection. 

 Among these enemies are some ani- 

 mals which have no means of piercing the shell, but 

 must watch their opportunity when the owner is quiet- 

 ly feeding, or so far extended that he cannot retreat 

 before the fatal blow is given. No one can have picked 

 up many spiral shells on the shore without noticing that 

 several of them were tenanted, not by the proper owner 

 of the shell, but by a kind of Crab, which has taken up 

 his abode in " the hollow-wreathed chamber." These 

 Crabs belong to a peculiar genus, called Pagurus, or the 

 Hermit Crabs, which are obviously fitted by Nature for 

 such a life, and unsuited to any other; and the Pagurus 

 would find himself quite as much inconvenienced by the 

 loss of his stolen coat as the natural owner himself. 

 We may, therefore, wonder that Nature should have left 

 him so unprovided as to subject him to the necessity of 

 feloniously appropriating the goods of another. But, it 



