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-like 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 undergone 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 to the present, although its importance in the life of this animal would appear 
to be great. 
This interlock (fig. 1,3, and 4, pi. xxxvii) is a simple but effective adjustment by 
means of which it is impossible for an enemy to pull out or twist off 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 1 , s 3 ) 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 find the hinges strengthened in a degree by the interlock of distinct but different 
spines (fig. 8-10, r 4 , and r 2 ), 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 
