196 THE LIFE OF CRUSTACEA 



latro Plate XXVII.) also belongs to the family 

 Ccenobitidse, and has attracted much notice from its 

 relatively gigantic size and its singular habits. Al- 

 though resembling Ccenobita closely in essential 

 structure, Birgus differs from it and from most other 

 Hermit Crabs in not making use of a portable 

 shelter, perhaps owing to the difficulty of obtaining 

 one of suitable size. The necessary protection for 

 the abdomen is obtained by a redevelopment of the 

 shelly plates (terga) on the upper surface of the 

 abdominal somites. The abdomen is carried doubled 

 underneath the body to protect the soft under- 

 surface, and the animal, when threatened, seeks a 

 shelter for its vulnerable hinder part in the nearest 

 hole or cranny. The swimmerets are absent in the 

 male sex, and are present only on one side of 

 the abdomen in the female. This unsymmetrical 

 development of the appendages is interesting as 

 indicating the derivation of the Robber Crab from 

 ancestors adapted to living in the unsymmetrical 

 shells of Gasteropod Molluscs. The last pair of 

 abdominal appendages, which in other Hermit 

 Crabs serve to hold the body in the shell, are here 

 much reduced in size, and quite useless for that 

 purpose. The carapace is very broad posteriorly, 

 owing to the great development of the branchial 

 cavities, which are much too capacious for the very 

 small gills. As in the true Land Crabs, the lining 

 membrane of the gill cavity is thick and spongy, and 



