TORSION OF THE CRUSTACEAN LIMB. 



133 



The large forceps of either side differs in still other respects 

 from the more primitive type-form preserved in the slender feet. 

 Thus the lower margin of the weaker claw of the small cheliped 

 is incurved, as contrasted with the rotund or convex outlines 

 of the big chelae, a condition probably determined by the greater 

 muscular development of the latter. 



Fig. 3 



FIG. 3. Illustrating successive positions (a, b, t) in the rotation of the great 

 claws of the lobster, and further by the vertical projection (', />', c l ), shown below. 

 In a, a 1 , which represents the normal position in the larval lobster and adult crab, the 

 upper or anterior surface of the claw becomes, in the adult lobster the underside f, t\ 

 F, propodite. 



In Figs. I and 2, which represent the first and third claw- 

 feet of the left side as seen from above, with the natural fore- 

 shortenings, pins are inserted at successive hinge -joints, to illus- 

 trate the various degrees of torsion which parts of the limb have 

 undergone. The joints mainly affected are the carpodite and 

 propodite (Cp, P), and the extent to which they have been 

 twisted may be measured by a comparison of pins (or hinge- 

 joints) marked I and 2 in the figures. This phenomenon may 



