SEA-FISHERIES LABORATORY. 335 



membranous plate divided into a dorsal and a ventral 

 portion. The dorsal portion lias on its inner side a down- 

 wardly projecting process which fuses with the upper 

 part of the second thoracic endosternite (,/'.) as described 

 above. The ventral portion of the endopleurite has an 

 upper crescent-shaped region and n lower part which 

 fuses with the endosternite. 



From the posterior side of the lower portion of the 

 endopleurite is given off a rod-like process which fuses 

 with the first thoracic endosternite. 



First post-oral cephalic arthrophragm (Text fig. 

 7, A.). In addition to the portion fused with the last 

 cephalic endosternite, the endosternite of the above 

 arthrophragm lias an anterior process at each side which 

 form the skeleton of the metastoma (PI. Ill, fig. 18, met.) 

 or posterior lip of the mouth. 



The endopleurite arises from the soft membranous 

 epimeron immediately behind the insertion of the external 

 abductor muscle of the mandible. It passes backwards 

 and gives rise to a small upwardly directed process, and 

 afterwards becomes joined to the fused endosternites. 



Ixtegitmext (Text fig. 8). 



The crab is covered by a continuous chitinous 

 exoskeleton, which serves partly as a protective covering 

 and also as a means of attachment for the muscles. The 

 main portion of this exoskeleton is strongly calcified. 

 Between the movable somites of the abdomen, however, 

 and also between the articulating segments of the 

 appendages, the exoskeleton remains uncalcified in order 

 to allow of free movement, ami has the appearance of a 

 thin chitinous membrane, known as the " arthrodial 

 membrane/' 



The exoskeleton of the ventral region of the abdomen 



