1914-15.] The Reflexes of Autotomy in Decapods. 251 
year, the percentage is found to be much lower, and rarely goes above 
30 per cent. 
In the summer of 1914 (August) collections were made again at Lion 
Rock, Cumbrae, and it was found that the percentage of crabs showing 
signs of having lost legs was in all cases below 35 per cent, (for three- 
year-olds). 
There are, therefore, two factors at work in causing autotomy : one 
is the nature of the shore, and the other is the season of the year. The 
storms of winter doubtless cause great movement of stones under which 
the crabs live, and thus limbs must be liable to crushing. On sandy shores 
there will, of course, be very much less danger. The moulting season is 
in early summer, and by August limbs lost during the winter have 
regenerated. 
Shore crabs live peaceably together even though fifty of various sizes 
are kept in one tank. Only one autotomised leg may be found in weeks. 
In the natural state, therefore, the chief producer of autotomy in Carcinus 
is crushing by movement of stones. Thus self-amputation is more a 
provision for stoppage of haemorrhage than for escape, for it allows the 
valvular mechanism at the breaking-plane to close. The same applies to 
the other Brachyura, but in these the loss of limbs is not so great, for 
they are not permanent shore-dwellers. Cancer, Hyas, and Fortunus 
show a 10 per cent, figure for loss of limbs. 
In a typical crab autotomy is purely a unisegmental reflex. Only in 
one form does plucking with the chelae ever take place (in the spider crabs), 
and this is due to the lack of carapace overhanging the basal parts of the 
limb, which is essential for the process. In Cancer, Carcinus, and Fortunus, 
after a limb is crushed or cut across it immediately extends violently at 
the second joint, and, coming in contact with the carapace, snaps cleanly 
across at the breaking-plane in the basi-ischium. In Fortunus, however, 
autotomy may occur without the limb coming in contact with external 
resistance. This resembles the condition found in Galatheids. A crab 
will keep on autotomising limbs as they are cut, till none are left, and 
may not move more than half a yard from the spot on which it received 
its first cut. 
If a cut leg of Carcinus comes in contact, during extension, with a 
resistant body before it reaches the carapace, e.g. the finger of the operator, 
division takes place at the breaking-plane, just as it does when the limb 
is extended quite against the body of the animal. In all cases a point of 
resistance for the distal part of the limb is necessary. The same is 
necessary in the case of Cancer. In Hyas, however, a difference is some- 
