304 Dzi'cllers of the Sea and Shore 



hermit crabs, the PyJochelidce. I have elsewhere in this 

 work shown by internal evidence that the little hermit 

 crab, Pagurus, of our shores, at one stage of its evolu- 

 tion was the possessor of a lobsterlike hind body. 

 Well, here, in this deep-sea form, is one whose hind 

 body is segmented, perfectly symmetrical, and shows 

 no indication of ever having acquired the habit of liv- 

 ing in gasteropod shells. The truth is, these forms are 

 so primitive that it is somewhat difficult to find many 

 features by which to distinguish them from the true 

 lobsters themselves. No fossil representatives of the 

 hermit crab have yet been found; there can be scarcely 

 any doubt that the Pylochelida^ are closely connected 

 with the primitive stock from which other hermit crabs 

 evolved. After all, what seems most remarkable is, not 

 that the abyssal crustaceans — and all other deep-sea 

 animals, for that matter — are dissimilar to those of 

 shallow water, but that they differ so slightly. Cer- 

 tainly, it should seem from the physical conditions pre- 

 vailing in those great depths, the freezing cold, the inky 

 darkness, and the stupendous pressures, that their re- 

 actions would be strikingly different from those of crea- 

 tures inhabiting the shallow water; but, strangely, the 

 differences of function are in very numerous instances 

 accompanied merely by the most trivial differences in 

 structure. 



From the fact that some of the free-swimming crus- 

 taceans of the deep sea have been found at, or near, the 

 surface, it has been supposed that they may do so for 

 the purpose of spawning, w^hich is not unlikely, con- 

 sidering the sensitiveness of young animals to cold ; but, 

 as shall later be pointed out, other urgent factors bring 



