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1914-15.] The Eeflexes of Autotomy in Decapods. 
regeneration occurs very much more easily from the region of the breaking- 
plane than from any other level. Morgan (1) has pointed out this fact in 
his experiments on hermit-crabs, though at the same time he indicates 
that slower and less perfect regeneration is possible from all levels, both 
above and below the breaking-plane. In typical regeneration a lirnh-bud 
is first formed, many times smaller than the normal leg. In this papilla, 
as it is called, differentiation goes on, and all the parts are laid down in 
miniature. On the occurrence of moulting, sudden increase of blood- 
pressure causes great expansion of the miniature limb, and in a few days 
the leg is ready for service with its fellows. Calcium salts are deposited 
in its integument at the same rate as in the rest of the hard integument ; 
and after the new muscles have gone through certain chemical changes, 
only slight difference of size enables an observer to distinguish it from the 
other legs. The small size of the regenerating appendage during the 
period in which it is functionless and tender is a marked benefit to the 
animal, for further damage is less likely to happen to the papilla than it 
would to a leg regenerating at once to normal size. 
One other benefit comes to the animal by its sacrifice, and this is as 
vital to the welfare of the crab as escape from enemies — fatal haemorrhage 
is prevented. The crustacean blood circulation in a limb differs from that 
seen in the higher animals. Blood passes by means of arteries to the distal 
end, but the venous channels do not confine it to such strict limits. They 
are broad sinuses occupying all the limb space not filled by muscle or 
dermis. Thus crushing of the shell-like limb causes the crab to run a 
grave risk of death by haemorrhage. But provision is made for this con- 
tingency. At the breaking-plane a diaphragm or “ membrane obturatrice ” 
(Fredericq (2) ) stretches across the lumen of the limb, and a foramen in 
this gives passage to the nerve and artery. This diaphragm consists of 
two flaps, and when autotomy occurs these are forced together by relative 
change of pressure on the outer side. They therefore act as a venous- 
valve, and the moment autotomy takes place bleeding is stopped. The 
mechanism is fully described in the case of the hermit crab in a previous 
paper by the writer (15). 
Both this valvular mechanism and the power of autotomy are very 
well developed in the shore-crab {Garcinus moenas (Penn.)). This animal 
uses them to good purpose, for, living on the shore between tide marks, 
and usually on stony ground, it is subject to the danger of crushing from 
movement of the stones by violent seas. So much is this the case, that 
after the winter storms fifty per cent, of the crabs collected are found to 
have autotomised one or more limbs. 
