120 THE BIOLOGY OF THE SEA-SHORE 



asunder, the result being to cause great weakening of the limb 

 at the breaking furrow. If the limb is now flexed its distal 

 portion will be brought into contact with the carapace, or 

 possibly with some external point of resistance, when the slight 

 lateral force will be sufficient to cause the already weakened 

 limb to snap. In Portuntis no lateral resistance is required, 

 the antagonistic action of short and long extensors being 

 sufficient of itself to cause splitting at the breaking plane.' 



.^^ 



Je ' S.e. 



Fig. io. — Au to tomy of crab's leg. 5.e.,L.e., short and long extensor 

 muscles ; S.f.,L.f., short and long flexors ; B.p., breaking plane (after Paul). 



In a typical crab autotomy is a purely unisegmental 

 reflex action, that is to say, it has its centre in a restricted 

 area of the nerve cord, and can take place when the nerve 

 cord is severed both above and below this area. It is only 

 in the spider crabs that imperfectly successful autotomy 

 may be completed by plucking with the forceps, and that 

 higher nerve centres consequently come into action. 



One of the most remarkable circumstances associated 

 with the phenomenon of autotomy is the occurrence of a 

 membrane or diaphragm the function of which seems almost 

 certainly to be that of preventing haemorrhage. The 

 diaphragm takes the form of two flaps which stretch across 

 the limb cavity from either side and appear to be formed 

 from modified epidermal cells at the breaking furrow. It 

 is pierced by a pair of foramina which allow for the passage 

 of artery and nerve. Immediately after autotomy the crab 

 extends the stump so that the diaphragm faces upwards. 

 " The artery retracts and its torn end dilates, forming a 

 little aneurism filled with plasma and lined with blood cells. 

 The nerve also retracts, and the funnel-like flaps of the 



