140 The American Naturalist. [February 
till the reaction produces strain enough to rupture the second 
segment (s) at the groove or weakest point (a). 
Some observations made upon the region where autotomy 
takes place, or plane of rupture as we may call it, seem of inter- 
est in supplementing the above account from an anatomical 
" point of view. In the crab Libinia the 
Æ, Chitinous wall of the limbs is exceeding 
(:J# thick and strong, so that to break it at all 
=} —and it will not break with a clear-cut 
fracture—considerable force is required, so 
that it seemed to me quite puzzling that the 
crab could throw off its legs with a smooth, 
clear-cut fracture, and this by the applica- 
tion of some force acting inside a firm 
cylinder. 
Experiments made were confirmatory of 
the facts demonstrated by Léon Fredericq— 
that autotomy is a reflex act, and that ap- 
parently and probably it is brought about 
by muscular contraction forcing the limb 
against the thorax. 
Examination of the second segment 
F; gd shows these two grooves around it (/ and 
d, Fig. 2) differing in appearance, but both conspicuous from the 
absence of hairs and hair pores in the exoskeleton along these 
lines. When autotomy takes place the limb separates along the 
proximal line ( ), and the exposed edges of the exoskeleton are 
smooth and sharp cut. The ex- 
posed surface of the soft central 
part of the limb is covered by a 
firm membrane except at the 
Fu. 2 centre, where there is a rounded 
I hole with a little torn tissue and 
blood exposed. After a time a chitinous layer appears over the 
end of the stump or short ring remaining as representative of the 
second segment. Though artificial rupture cannot ordinarily be 
brought about at this “plane of rupture,” yet when the limb is 


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