198 EDMUND B. WILSON. 



As shown in Fig. I, A, B, the chelae in this species differ 

 very widely both in size and structure ; right-handed and left- 

 handed individuals occur in approximately equal numbers. The 

 large or hammer chela (/>), which is nearly or quite half the length 

 of the whole animal exclusive of the antenna, is greatly swollen, 

 with a deep transverse groove on either side of the propodus, and a 

 characteristic color-pattern. Both claws are extremely stout, and 

 show very remarkable and characteristic features. On the con- 

 cave side of the dactyl us is a great swollen knob, forming the 

 " hammer" (//), which fits into a corresponding deep socket on 

 the outer side of the propodus claw, overhung on the upper side 

 by a prominent setose ridge (/. .$.). By fully extending the dac- 

 tylus and then suddenly snapping the claws together the hammer 

 is forced into the socket with such force as to produce a surpris- 

 ingly loud report, whence the popular name " pistol crab " ap- 

 plied to the animal in some localities (cf. Brooks and Herrick). 



The large chela has essentially the same structure in both 

 sexes, but the small one shows characteristic sexual differences. 

 In the female (Fig. 2, D) it is very straight and slender, and rela- 

 tively smaller than in the male. In the latter (Fig. 2, A) it is 

 not only relatively somewhat larger, but stouter, with relatively 

 shorter claws, the dactylus more strongly curved, and traces 

 of the transverse grooves of the large chela are often present 

 (Fig. i, A). Its most characteristic feature in the male is the 

 presence on both sides of the dactylus of a very marked curved 

 ridge, bearing a series of stiff, short setae (d. s.). A somewhat 

 similar but straighter setose ridge (/>. s.} is also present on each 

 side of the propodus claw in a position corresponding to that of 

 the setose ridge in the large claw. No trace of the hammer 

 is present in either sex. Ordinarily the small chela alone is 

 used in taking food, the large one being in the main a weapon 

 of offense and defense, as Brooks has graphically described. 



As far as the reversal of asymmetry is concerned my observa- 

 tions on A. JictcrocJielis entirely confirm those of Przibram on the 

 three species studied by him, but give a slightly different result 

 in cases where both chelae are amputated. Przibram found (op. 

 cit., p. 331) that in such cases each stump regenerates an appen- 

 dage of the same type as that which has been removed (/. e., no 



