249 
1914-15.] The Keflexes of Autotomy in Decapods. 
merits is a very effective swimming organ. The two types examined most 
carefully were G. squamifera, Leach, and G. dispersa, Bate. These are 
found on the shore usually clinging to the underside of stones. In spite of 
the fact that autotomy occurs very readily, the percentage of Galatheids 
taken short of legs is comparatively small, probably between 5 per cent, 
and 10 per cent. 
The first observations made on autotomy are usually the result of 
accident, for in shore collecting nearly every Galatheid picked up by a 
cheliped or walking-leg promptly abandons the leg and drops back into 
the weed. One would be very much inclined to give the animal credit 
for deciding that it would be better to escape without a leg than be 
taken. So much is this the case that Pieron (10) recently, in describing 
a similar form of autotomy in Grapsus, calls it psychic. The evidence 
tends to show that no decapod limb is abandoned except as the result of 
some stimulus received in that limb, which is nocuous to the animal. 
This is indicated by the fact that the ordinary porcelain crab (Porcellana 
platycheles), when suspended by the hydroid which grows on its claws, 
will not autotomise. If it is picked up on the beach by the fingers, ever 
so lightly applied, autotomy almost invariably happens if a leg be held. 
It is not clear whether pressure or whether the heat of the fingers is the 
nocuous stimulus. In any case it rarely happens in the laboratory (where 
the heat of air and wafer must be nearer that of the body). M. Pieron 
records the same fact for Grapsus. 
If limbs are sharply cut by scissors, the Galatheid has usually shot 
backwards by tail movement before the scissors are disengaged, and the 
limb is broken off at the breaking-plane. If the cut be clean enough, 
however, the limb is often shot off by the animal in the course of its 
flight, without touching any point of resistance. At other times it is 
retained, and may be later plucked off, either by the chelipeds or by the 
diminished fourth leg, which is usually used for cleaning. The leg is 
usually markedly extended at the second joint, and, if it is touched by the 
Anger or forceps, usually snaps off at the breaking-plane. In comparison 
with the Pagurids, it may be said that the local weakening of the limb at 
the breaking-plane by segmental reflex muscular contraction is very much 
more marked. 
Escape seems to be the chief purpose of autotomy in the Galatheids, for 
movements of flight always accompany self -amputation. 
In the Brachyura, which are studied in the next section, stoppage of 
haemorrhage seems to be the main purpose, for usually the animal does not 
attempt to fly from the source of danger. 
