100 BRANCH ARTHROPODA 



Hermit Crabs (Fig. 73). — There are a number of species of 

 hermit crabs which are not true crabs, but are more Hke the 

 lobster and crayfish. They have the habit of backing into 

 empty univalve shells which they carry about with them and 

 into which they may withdraw for protection. This habit has 

 resulted in a soft-skinned, reduced abdomen, with a spiral 

 twist and with no appendages except a pair of hooks for hold- 

 ing on to the inside of the shell. The abdomen is always hitlden 

 in the shell. The head, thorax, and legs project when the 

 animal is active, but are withdrawn when danger approaches. 



Fig. 73. — Hermit crab {Pagii'rus) in shell, with a sea-anemone {Adam'sia 

 pallia'ta) attached to the shell. (After Hertwig.) (From Jordan and Kel- 

 logg, " Animal Life," D. Appleton and Co., Publishers.) 



As it grows it discards its shell and hunts a larger one. Some 

 of these hermit crabs have a peculiar commensal life with cer- 

 tain sea-anemones (Fig. 73), which they carry about on their 

 shells. If the sea-anemone becomes detached the crab hunts 

 another and places it on its shell. The crab is protected from 

 its enemies by the stinging threads of the anemone, also by 

 its resemblance to the seaweed, while the anemone is assured 

 of a fresh food supply by being carried from place to place by 

 the crab. 



Crabs are other examples of this order. The cephalothorax 



