50 



tend to pull different segments of the limb in directions shown 

 in figs. 16 and 17. The result, of course, must be great weakening 

 along the breaking furrow, and if a slight force is apphed laterally 

 the portion of the limb distal to the furrow snaps off. 



To prove the above, a knife was inserted firmly between the 

 insertions of the extensors, forcing the segments apart as indicated 

 in fig. 18. The limb snapped off on the slightest lateral force 

 being apphed. When the knife was not inserted no amount of 

 twisting and pulhng could accomplish division at the seat of 

 election. 



That the extensor muscles contract separately and with shghtly 

 different latent periods is shown by figs. 19 and 20. 



Thus, autotomy in the Brachyura is seen to be due to 

 co-ordinate action of the basal muscles attached to the second 

 segment, and to follow weakening in the region of the breaking 

 plane. It is the result of a unisegmental reflex, and its chief 

 purpose is to prevent haemorrhage. 



It is interesting to note the points of similarity and difference 

 between this peculiar reaction to injurj^ in two large branches of 

 the Decapoda, like the Macrura and Brach^^ura, and to examine 

 the effects of habits and habitat on the behaviour of the animals. 



In the lobster, where the natural loss of legs is comparatively 

 small, the act of autotomy is pluri-segmental, and resembles closely 

 the animal's ordinary procedure when escaping from an enemy. 

 Evasion is therefore its main purpose. In crabs, where the reflex 

 arc is confined to one segment of the nervous system, hsemostasis 

 seems to be the chief end. The liabilit}" to crushing of limbs on 

 a stony shore is evidently great, for as a rule half the mature crabs 

 collected in certain places have autotomised. 



This tendency to locahsation of mechanisms for protection in 

 one part of the nervous system is a general principle of animal life, 

 and the more a reaction tends to preserve the life of an individual, 

 the more is it liable to be so localised. The benefit attained is, of 

 course, more speed in performance of the act. For example, in 

 the lobster, where impulses must travel through several neurons 

 to ganglia controlling the tail, the latent period for reaction is 

 about four seconds. In the highly specialised Brachyura, where 

 liabihty to injury is very much greater, only a fraction of a second 

 ntervenes between injury and autotomy. 



