51 



From a morphological view-point, too, there is interesting 

 difference between the mechanisms of the two groups. 



Lobsters effect the amputation by weakening of the limb at 

 the breaking-plane, but removal is partly by snapping of a brittle 

 substance, and partly by tearing through tough membrane. This, 

 of course, pre-supposes a traction force on the abandoned part of 

 the limb. Crabs, on the other hand, so weaken the region of the 

 breaking-plane that the least force applied either by holding or by 

 contact with an external object immediately causes the leg to 

 snap off, the break, in all cases, being entirely through brittle 

 integument, giving efficient support to the valve flaps which imme- 

 diately come into action.* 



Much of the observation connected with the above work was 

 carried out at the Dove Laboratory, Cullercoats, and I have to 

 thank Professor Meek for the facilities he has given me for working 

 there. The rest of the work w^as done at the Marine Biological 

 Station, Millport, and there Mr. R. Elmhirst extended to me his 

 usual courtesy. Mr. J. Peden also gave valuable help in the 

 making of records. 



Professor Xoel Paton, under whose supervision I have worked 

 for some time, constantly encouraged and aided me in my experi- 

 ments, and to him, again, I tender my sincere thanks. 



SUMMARY. 



(1) Autotomy of the walking-legs in the lobster and its allies 

 is not a simple tearing at the third joint as other writers have stated, 

 but is the result of a pluri-segmental reflex, which, among other 

 things, causes great weakening of the hmb at the central end of 

 the third poclomere along a groove running partly round the limb 

 at this point. 



(2) This w^eakening is caused by a definite muscle which is 

 described. 



(3) Autotomy of walking-legs in the lobster occurs about four 

 seconds after nocuous stimulation. In the Brachyura there is a 

 latent period of only a fraction of a second. 



* The existence cf a breaking-plane as a structural entity has not been forgotten, and I 

 intend to investigate its comparative morphology. 



