Crustacea. 



Blind Crayfishes. In the Mammoth Cave, and some other caves, 

 are found blind crayfishes. This does not mean that in all cases eyes 

 are completely lacking, in fact, in most cases rudiments of eyes are 

 present, but useless. How long these animals have thus lived in dark- 

 ness we do not know ; but we find that an organ that is no longer used 

 may lose its function and even dwindle away. We here see illustrations 

 of the general law that disuse leads to deterioration in both structure 

 and function, often resulting in complete uselessness, and perhaps com- 

 plete atrophy as well. 



Hermit Crabs. These crabs back into an empty univalve shell, which 

 they carry around with them for protection. The abdomen, and all the 



FIG. 53. BLIND CRAYFISH OF MAMMOTH CAVE, NATURAL SIZE. 



parts except the head and projecting appendages, become soft, and de- 

 pendent upon a continuance of such protection. When the crab gets 

 too big for the shell, he hunts for a bigger one. It is said that these 

 crabs sometimes fight over shells. One hermit meeting another crab 

 that has a shell that he thinks would fit him better than the one he has, 

 ejects the other fellow, perhaps only to find that he has gotten a misfit 

 after all, and so goes back to his old shell, a wiser if not a better crab. 

 Barnacles. If one lies down on the edge of almost any stone pier 

 or wharf along the coast and looks down into the water, he may see a 



