NATURAL HISTORY OF AMERICAN LOBSTER. 259 



limb we have an illustration of an adaptive variation, which in origin and the extent 

 to which the process may be carried is independent of use and the mechanical strains to 

 which the organ may be subjected. 



Apart from their crushing or piercing teeth and sharp indurated tips, the large claws 

 are armed along their facing edges by stout tooth-Uke spines, while the exposed sur- 

 faces and angles of the lower segments of the limb are similarly protected. These spines 

 are generally directed forward and mostly upward and tend to guard the space about 

 the head which the outstretched claws inclose (see, p. 273). 



The terminal segments of the last pair of slender legs have imdergone torsion but of 

 a different character, as described in chapter ix, page 304. 



BREAKING PLANE AND INTERLOCK. 



We have seen that both of the large chelipeds have a stiff or breaking joint in the 

 compound segment at their base, as well as peculiar hinges, which are not only adapted 

 to the ordinary uses of such limbs, but possibly to the resources of the animal in sacrific- 

 ing them for its own preservation. There has also been developed in relation to the 

 breaking plane an interesting interlocking mechanism, which seems to have escaped 

 notice up tothe present, although its importance in the life of this animal would appear 

 to be great. 



This interlock (fig. 1,3, and 4, pi. xxxvir) is a simple but effective adjustment by 

 means of which it is impossible for an enemy to pull out or twist o£f one of the chelipeds, 

 as may be done in a cooked lobster, without bringing autotomy into play, to which 

 process it seems to form a sort of emergency "brake." 



Turning the body of the lobster over and working the chelipeds by hand, we per- 

 ceive that they move freely forward and backward, the striking or thrust movement, 

 at the junction of coxa with basis. In such movements the lobster's most powerful 

 blows are dealt, whether in attack or defense. We observe further that any lateral 

 movement of this joint would be serious, and that is guarded against by huge inter- 

 locking spurs (s^, s^) on the first and third podomeres respectively. This condition 

 seems to be related to the fact that the breaking joint (x) lies between these points, 

 or peripheral to a free joint, so that when the strain upon this articulation and the inter- 

 locking spurs is too great or, in other words, sufficient, the limb is reflexly cast off in the 

 breaking plane. 



This mechanism, moreover, together with the complete fusion of the joint, is not 

 developed until after the fourth stage, when there is probably less need of strengthen- 

 ing the hinges between these particular segments. Yet autotomy occurs at this stage, 

 and we fin d the hinges strengthened in a degree by the interlock of distinct but diifferent 

 spines (fig. 8-10, s*, and s'), although this early adjustment is not quite so marked 

 as in the adult animal. At all events in the lobsterling there is an interlock between 

 the second and third podomeres, which evidently increases the resistance of the limb 

 at its base during this period. These spurs of the fourth stage lobster become later 

 reduced to rudiments, and new interlocking processes are developed in the adult animal 

 62399° — II 8 



