REGENERATION 



17 



this remarkable power in crabs, and described the mechanism on 

 which it depends. When the leg is irritated, for instance when it is 

 pinched at the tip and held fast, it breaks off at a particular place. 

 This line of breakage lies in the middle of the short second joint 

 (Fig. 98, J. and B, s), just between the insertions of the muscles 

 (me, mf, m) which extend from this line towards the extremity of 

 the limb and in the opposite direction towards the body- wall. 

 Between these muscle-attachments the external skeleton is thin and 

 brittle, and forms a suture, s, which 

 breaks through when the animal con- 

 tracts the muscles of the leg convulsively, 

 and thus presses the lower protuberance 

 (a) against a projection (b) of the first 

 upper joint. Crabs require to make 

 a verj' considerable muscular exertion 

 before they can throw ofi" the limb, and 

 therefore they can only do it when they 

 are in full vigour. 



We have here a quite definite 

 structural adaptation of the parts to 

 a danger which often recurs — that of 

 falling entirely into the power of an 

 enemy which has seized the leg. By 

 a sudden violent throwing-off of the 

 leg the crab escapes from this danger. 

 Quite similar adaptations are found 

 among certain insects, such as the 

 walking-stick insects or Phasmids, in 

 which the mechanism is much the same, 

 and lies at an almost exactly corre- 

 sponding place, namely, at the line 

 where the second and third joints of 

 the leg, the ' trochanter ' and the ' femur,' 



meet. In this case the advantage of the arrangement is not merely 

 that the animals are thus enabled to escape from enemies ; it is 

 useful in another connexion, for a knowledge of which we have to 

 thank Bordage. This naturalist observed that the Phasmids not 

 infrequently perished at one of their numerous moultings, by remain- 

 ing partially fixed in the discarded husk. Of 100 Phasmids nine 

 died in this way, twenty-two got free with the loss of one or more 

 legs, and only sixty-nine survived the moult without any loss 

 at all. 



II. c 



Fig. 98. The leg of a Crab, 

 adapted for self-mutilation or au- 

 totomy. A, the first three joints 

 of the limb, I, II, III. s, the 

 suture, that is, a thin area on 

 the second joint which is pre- 

 disposed to breakage, mf, flexor 

 muscle, me, extensor muscle, both 

 inserted at the suture. B the 

 entire leg with its six joints and 

 with the suture (s). Slightly en- 

 larged. After MacCuUock. 



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