A U TO TOM Y 



on the outside of the leg, a ring-like groove that marks the place of 

 rupture. A comparison of the legs of the crab with the walking 

 legs of the crayfish, or of the lobster, shows that the groove in the 

 crab's legs corresponds to a joint in the legs of the two other forms. 

 In the crayfish and lobster the walking legs generally break off at 

 this same level, although by no means as easily or with as much 

 certainty as in the crab. The first pair of legs of the crayfish and 



FlG. 45. A. After Andrews. Base of leg of crab to show breaking-joint, i-x. B. After Fre- 

 dericq. Diagram of leg of crab to show how autotomy takes place. C. After Andrews. 

 Longitudinal section of base of leg to show in-turned chitinous plate at breaking-joint. 



lobster, carrying the large claws, have also a breaking-joint at the 

 base of the leg similar to that in the crab's leg, and these legs break 

 off in the living animal always at the breaking-joint. 



Reaumur first recorded that if the leg of a crayfish or of a crab is 

 cut off outside of the breaking-joint it is always thrown off later at 

 the breaking-joint. Fredericq has made a careful examination of the 

 way in which the leg is thrown off in the crab, Carcinus manas. 

 He found that the breaking does not take place at the weakest part 



