56 TnE OCEAN. 



appearance crawling under the perpetual shadow of its 

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

 probably more than compensated by the protection 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 shell, 

 into which it thrusts its abdomen, henceforth using it 

 as a house. Now there is a species of Sponge found on 

 our coast [Halicliondria suherea), of a corky substance, 

 which grows on the surface of similar shells, over- 

 spreading and enveloping them ; and it so happens 

 that in the great majority of instances, if not in all, 

 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 l\is 

 premises becoming every day more and more -con- 

 tracted, is at length compelled to seek another lodging. 

 A proceeding very similar to this, but which the 

 Hermit crab finds rather to his advantage than dis- 

 comfort, takes place in the growth of a species of 

 incrusting Polype called Hydractima echinata. * This 

 Polype also very frequently 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 



* For a description of this highly curidus little Zoophyte, see 

 '' Evenings at the Microscope," p. 402. 



