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 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 
