EXTERNAL CHARACTERS 127 



junction with the pleuron. This peg fits into a 

 corresponding depression in the hinder margin of 

 the segment next in front, preventing movement 

 of the abdomen from side to side. 



2. The appendages are attached to the sides of the ventral 

 surface of the segment, between the epimera and 

 the ends of the sternum, by soft patches of cuticle, 

 the arthrodial membranes. Each appendage con- 

 sists of a two-jointed basal portion, the protopodite, 

 supporting distally two many -jointed filaments, the 

 endopodite and the exopodite, the former of which is 

 nearer the median plane. 



B. The Body. 



This is obviously composed of two parts, the cephalothorax 

 and the abdomen. 



1. The cephalothorax is composed of thirteen segments 

 fused together, the fusion being so complete that 

 the segmentation is not recognisable on the dorsal 

 and lateral surfaces, though it can still be seen 

 below. 



The cephalothorax is encased laterally and 

 dorsally by a large shield, the carapace. This is 

 divided into cephalic and thoracic portions, covering 

 respectively the head and the thorax of the animal, 

 by the shallow cervical groove, which runs trans- 

 versely across its dorsal surface, and obliquely for- 

 wards at the sides. 



a. The head, or part in front of the cervical groove, 

 is composed of five segments. In it are to be 

 noticed the following structures. 

 i. The rostrum is a median pointed prolongation 

 of the carapace, extending forwards from its 

 anterior end. 

 ii. The eyes are paired and mounted on movable 



stalks, one at each side of the rostrum, 

 iii. The mouth is far back on the ventral surface of 

 the head, and almost hidden by the jaws. 



