HALICHONDRIA. 141 



the period of its being quitted by one inhabitant, and the pos- 

 session of another ; for the Hermit-crab is continually changing 

 its abode, in order to accommodate its cell to its growth. 



" In every instance where a crab has been found to inhabit 

 such a shell, the sponge has invariably spread within the aper- 

 ture, and frequently a considerable extension of the lip to the 

 shell is produced by the sponge." — Montagu. 



The shelly nucleus of this sponge will often be found, on a 



section of it, to have been more or less completely absorbed, as 



is obvious from the disappearance of some of the whorls, or the 



unusual thinness of their walls. 



I 



FiQ-. 14. 



Whether this absorption of the shell has been eifected by the 

 Hermit-crab or by the sponge may be questioned, but the crab, 

 in my opinion, is the agent, for shells inhabited by it, and free 

 of sponge, have been found in a similar state of decay. " It is 

 probable," says Mr J. E. Gray, " that some Bernard crabs 

 have also the faculty of dissolving shell, for it is not unusual 

 to find the long fusiform shells, such as Fusus, Fasciolaria, and 

 Turbinella, which are inhabited by these animals, with the in-^ 

 ner lip and great part of the pillar on the inside of the mouth 

 destroyed, so as to render the aperture much larger than usual. 

 I have never seen this erosion except in dead shells which had 

 been inhabited by Hennit crabs ; but it does not occur in all 

 that are so tenanted, for I have also observed these animals 



