472 Victor E. Emmel 



use the apparatus and the excellent material available at the 

 lobster hatchery. 



NORMAL DEVELOPMENT OF THE CHELAE 



A brief description of the normal development of the chelae 

 of the lobster may aid in understanding the nature of the present 

 experiments. 



In the adult lobster, one of the two chelae, either the right or the 

 left, is a rather long slender nipper claw, and the other is a larger 

 and more massive crusher. Each claw consists of a movable 

 jaw, the dactyl, and an immovable jaw or index. In very young 

 lobsters the right and left chelae appear alike. During the first 

 three larval stages they are embryonic in character. The claws 

 are relatively short and broad; the index is smaller than the dactyl 

 and both index and dactyl are beset with long hairs or bristles. 

 During the fourth and fifth stages, the claws have become long 

 and slender but are still alike. Characteristic tactile hairs and 

 pointed teeth appear and the claws now begin to resemble the 

 adult nipper type. 



' At about the sixth stage however a divergence in the differen- 

 tiation of the two chelae becomes apparent. In one of the chelae 

 the nipper characters continue to develop. This claw retains 

 the long slender form characteristic of the adult. Tactile hairs 

 are distributed in a dense fringe on each side of the dentate mar- 

 gin. The pointed cutting teeth are arranged in a linear series 

 for each jaw with the exception of a stout displaced tooth about 

 midway in the dentate margin of the index. In marked contrast 

 to this development of the nipper, the other claw becomes wider, 

 broad tubercle-like teeth develop, and the tactile hair of the nipper 

 type gradually disappears in successive moults. Thus the adult 

 crusher claw comes to be characterized by the almost entire absence 

 of tactile hairs, and the presence of broad crushing teeth; and by a 

 form larger and more massive than that of the nipper. The 

 final result is the establishment of the adult asymmetry. 



In the development of the lobster therefore there is a series of 

 larval and adolescent stages, in which there is a transition from 



