68 THE OCEAN. 



appearance crawling under the perpetual shadow of 

 its own tree, the burden of whose weight, however, 

 was probably more than compensated by the pro- 

 tection it afforded against enemies. 



A singular little creature, called the Hermit Crab 

 (Pagurus), the hinder part of whose body is unpro- 

 tected, except by a soft skin, is endowed with an 

 instinct which prompts it to seek some univalve 

 shells, into which it thrusts its abdomen, henceforth 

 using it as a house. Now there is a species of 

 Sponge found on our coast (H. suberea), of a corky 

 substance, which grows on the surface of similar 

 shells, overspreading and enveloping them; and it 

 so happens that in the great majority of instances, 

 the Sponge is found upon the individual shells in- 

 habited by the Hermit. Gradually and insensibly 

 the Sponge grows over the shell, and at length creeps 

 round the edge of the lip, and begins to line the 

 inside : the constant motion of the crab, who is very 

 active, retards the growth for a while, but eventually 

 the Sponge prevails, and the Hermit, finding his pre- 

 mises becoming every day more and. more contracted, 

 is at length compelled to seek another lodging. A 

 proceeding very similar to this, but which the Her- 

 mit Crab finds rather to his advantage than discom- 

 fort, takes place in the growth of a species of Coral 

 (Alcyonium echinatum). This coral also very fre- 

 quently grows on a shell selected for a habitation 

 by the little* crab; but as it grows, it does not line 

 the shell, but becomes moulded, as it were, to the 

 form of the enclosed animal, thus increasing the size 

 and commodiousness of the dwelling, and precluding 



