122 A NATURALIST ON DESERT ISLANDS. 



Cenobita diogenes is the only species of the genus which 

 is found in the West Indian region. The family to which 

 it belongs (Cenobitidce) includes also the robber-crabs, 

 (Birgus), and of the life-history of both very little appears 

 to be known. Yet if we could follow up the threads of 

 the evolution of either, we should surely have unfolded 

 before us a most absorbing piece of history. 



We have remarked before that these land hermit-crabs 

 have left their old dominion in the depths of the sea, 

 where the descendants of their infinitely ancient and long 

 extinct marine ancestors still exist under a multiplicity 

 of forms. As the result of dredging in the Gulf of Mexico 

 and the Caribbean Sea,* including parts adjacent to these 

 very islands, the staff of the " Blake " found thirty-eight 

 species of marine hermit-crabs, of which thirty-three 

 were new to science. 



Some of these are not symmetrical like C. diogenes, 

 but have a perfectly straight abdomen, and the proper 

 number of appendages on both sides of it ; while the 

 shelters to which others have recourse in hiding their soft 

 bodies are very varied. Pylocheles, for example, lodge them- 

 selves in holes of loose stones or in the central cavity of 

 siliceous sponges. To render their lodging still more 

 secure they hermetically close its orifice with their closely 

 approximated claws, at the same time reinforcing these 

 with the ends of their anterior walking legs. In 

 Xylopagurus, again, the abdomen is quite straight. It 

 lives in perforated pieces of wood, choosing straight 

 chambers open at each end. Instead of entering back- 

 wards into its dwelling like our friend diogenes, this 

 marine hermit-crab enters it directly, and then guards the 

 anterior orifice with its large claw, and the posterior one 

 with the sixth calcified segment of its abdomen. Pylo- 

 pagurus conforms more closely to the present land type 

 we have been describing, and chooses univalve shells 



* Reports on the Results of Dredging in the Gulf of Mexico and Carib- 

 bean S'Pia by the U S. coast survey steamer " Blake," Descriptions of 

 the PaguridcB by Milne-Edwards and Bouvier. 



