CRUSTACEA 



167 



use its abdomen, the larva changes rapidly to the adult 

 crab form. The unused abdomen is tucked awa}^ the cara- 

 pace broadens, and the walking legs speedily develop. Having 

 acquired the adult form, the little crab moults its shell 

 regularly, and it is interesting to collect the whole series of 

 shells obtained from a single shore crab. 



Such a series is shown in Fig. 102, which is a photograph 

 of specimens obtained from a single crab between May 1901 

 and July 1904, and exhibited in the British Museum. 



Fiu. 103. A Hermit Crab (I'agurus), inhabiting a Whelk Shell. 



Hermit 

 Crabs. 



The Hermit Crab (Pagurus) is peculiar because 

 of its custom of inhabiting some empty Gasteropod 

 shell, usually that of a whelk (Fig. 103). 



The hermit protrudes its head and great legs from the 

 shell, but tucks its soft, long abdomen away inside, twisting 

 it right round into the spiral cavity of the whelk shell. The 

 soft abdomen, protected in this way, is very unlike that of 

 other Crustacea, for not only has the hard exo- skeleton 

 disappeared, but it is also asymmetrical (Fig. 104). Many of 

 the abdominal appendages have disappeared, and those that 



