THE CRAYFISH 



327 



has two divisions. In the adult female the most anterior pair of 

 abdominal appendages are reduced in size and are uniramous. 

 In the larval stages of both sexes, however, they show the bi- 

 ramous structure, and by following their development it is seen 

 that the exopodite is lost during the late embryonic stages with the 

 result that only protopodite and endopodite remain. 



The structure of the pereiopods and chelce may be understood 

 even without a knowledge of the development, if one examines 

 first the third maxilliped (Fig. 157 D). This appendage is com- 

 posed of two parts: a protopodite, divided into a proximal por- 



Fk;. loS. — Young American L()l>.ster, Ifin/iariis (mierirnnus, at a stage when 



exopods are present on the pereiopods. 



(After Herrick, BuU. U. S. B. F., 1895.) 



tion bearing a gill, with which is fused a delicate outgrowth known 

 as an epipodite; and a distal portion which bears the exopodite 

 and endopodite. Of these, the exopodite is smaller and finger- 

 like, with small divisions at the distal end, while in the endopodite 

 five segments may be counted, the proximal one being fused with 

 the outer portion of the protopodite. By comparison with the 

 third maxilHpeds, it will be seen that the pereiopods and chelae 

 lack exopodites. In the embryonic stages, however, they 

 have exopodites somewhat in the manner of the larval lobster 

 shown in Fig. 158. These ^ve posterior appendages of the 

 thorax are, therefore, constructed according to the fundamental 



