6 THE FOUNDATIONS OF ZOOLOGY 



that has found or emptied a shell that seems to suit, measures it 

 carefully inside and out, and then, bringing the openings close 

 together, quickly pops out of the old into the new. Then the old 

 shell is compared with the new, and often the body is slipped 

 into each of them repeatedly, and each is allowed to slip nearly 

 off and is then pulled on again, somewhat as a man settles him- 

 self into a new coat. Running is now tried in each shell, a claw 

 keeping a tight clutch on the empty one and dragging it along; 

 and the movement of drawing the body far into the shell, so that 

 it drops on the sand as if it were empty, is tried in both. It is 

 often many hours before a choice is made, and then the decision 

 often is that the old one is best. 



It is difficult to witness or to describe this performance with- 

 out attributing to the crab feelings and motives like our own ; 

 yet, while no one can say whether the crab knows what it is 

 about or not, nothing is more certain than that its actions are 

 due to its nature, and not to knowledge of the value of a house, 

 drawn from experience. When I was working as a student in the 

 marine laboratory of Alexander Agassiz, he reared from eggs, in 

 an aquarium, a brood of hermit-crabs which had never seen a 

 shell. I had in my aquarium young gasteropods which I had 

 reared from eggs. Some of them had died, and their empty 

 shells were, at his suggestion, dropped into the water with the 

 crabs, which seized them, almost as soon as they touched the 

 water, and beginning to explore their interior as they were carried 

 to the bottom by the weight of the shells, conducted themselves 

 as if they had many years of experience in the use of molluscan 

 shells as houses. I have seen very young hermit-crabs make 

 houses for themselves out of the cast skins of others, although 

 these afforded no protection; and I have found a full-grown one 

 in the bowl of a clay pipe so badly broken that it exposed the 

 soft abdomen and was useless ; but the impulse to inhabit shells 

 is almost universally protective and beneficial, although it is as 

 strictly a part of the nature of hermit-crabs as is the twisted 

 abdomen, or the legs and claws, or any other part of the crab's 

 body. 



The external world presents such variety that few natural ad- 

 justments are so exact and definite that they may not under some 



