60 



Amongst the Brachyura two kinds of autotomy have 

 been recognised. 



(1) If the crab is captured by means of one of its 

 limbs it will throw off the limb in order to escape from 

 its enemy (" evasive autotomy "). 



It is evident that all Decapods do not act similarly 

 under such conditions. It does not appear to be the case 

 in Cancer or Carcinus. Fredericq's researches led him 

 to believe that crabs did not throw off legs in order to 

 escape from enemies, but his experiments were confined 

 to a few species. Taking all the evidence available, it 

 would appear that autotomy does take place under the 

 above conditions in some crabs, such as the Maiidae and 

 the Grrapsidae. 



(2) If one of the legs of a crab be severely wounded, 

 the limb will be thrown off. This probably occurs without 

 exception in the Brachyura. 



It is well to remember that in both cases we are 

 probably dealing with essentially similar physiological 

 conditions. In both cases the autotomy is produced as 

 the result of the stimulation of the nerve of the leg, and 

 the difference appears rather to be one of degree than of 

 kind. In both the above cases the autotomy is produced 

 as the result of a retiex, and the seat of this reflex is in the 

 ganglion of the somite to which the autotomised leg 

 belongs. 



Quite recently, Pieron* has concluded that there is 

 still another kind of autotomy which is purely voluntary, 

 and will not take place after the commissures connecting 

 the cerebral ganglia with the thoracic mass have been cut. 

 One of his experiments with Grrapsus was as follows : A 

 leg of the crab was tied to a stake within view of a 



* Pieron, H. C.R. Soc. Biol., llth May, 1907. Ibid., T. LXIII 

 (1907), Nos. 33 and 34. 



