DISTRIBUTION OF ORGANISMS. 175 



nation of these animals shows that they are adapted by special 

 peculiarities for this kind of life. In the common British 



FIG. 90. 



Hermit Crabs. 



A, Enclosed in shell of Whelk ; B, The animal removed from its 



protective covering. 



hermit-crab, the third and fourth pairs of locomotive limbs are 

 of small size, being buried wholly within the shell, where they 

 are applied to the columellar fold, as a means of fastening the 

 animal in the recess. Farther in, and also employed in fasten- 

 ing the body to the shell, is the caudal part, with two holders 

 developed for this express purpose, and as rough as a file. 

 The hold is still further secured in some species by rows of 

 suckers along the abdomen. Add to all this, that for want of 

 room at the mouth of the shell, only one of the pincer claws is 

 well developed, usually the right, while only the two front 

 pairs of feet are used for locomotion, and we see that, whether 

 we take these crabs as a species, a genus, or a family, their 

 ordinary form that thing which naturalists regard as immu- 

 table, and as originally the effect of a special creative effort 

 is in direct relation to the existence and forms of turbinate 

 shells formerly possessed by a different class of animals, 

 which must therefore have existed before the hermit-crabs. It 

 may be well to mark the credulity to which the adherents of 

 immutability must here be reduced. They must believe that 



