A LIMB FOR A LIFE 171 



parts of a ring of calcareous integument. The 

 amputation is more complex and more effective. 

 The autotomy does not work unless the limb be 

 gripped, but in some cases, again, the animal may 

 pull off a damaged limb with the help of one of the 

 clawed appendages. (4) In hermit-crabs, which 

 shelter their soft tail in a borrowed Gasteropod 

 shell, a damaged limb is amputated simultaneously 

 with the withdrawal within the sheltering shell. 

 A message travels to the nearest ganglion of the 

 ventral nerve-cord; an answer comes back com- 

 manding violent muscular contraction at the base of 

 the leg; and in a moment the limb is severed. But 

 it is very interesting to find that a hermit-crab upset 

 by being removed from its borrowed shell may pluck 

 at an injured limb with its forceps, or may even bite 

 it down to the breaking-plane, thus falling back on 

 autophagy. (5) It is in crabs that the autotomy 

 reaches perfection. There is a definite breaking- 

 plane, a line of weakness, across 1 the second basal 

 joint; the breakage is due to the forcible antag- 

 onism of muscles working at this plane; the snap 

 occurs before one has time to say " self -amputa- 

 tion." 



In the shore-crab and the edible-crab the limb 

 cannot break off unless the distal part of it be pressed 

 against something, such as the animal's own shell 

 or a stone; in the swimming-crab and the sand-crab 

 even the point d'appui is dispensed with. But 

 perhaps the neatest adaptation in crabs is the dia- 

 phragm or bandage-membrane which stretches 



