EXTERNAL CHARACTERS. 135 



on the anterior margin of the tergum, near its 

 junction with the pleuron; This peg fits into a 

 corresponding depression in the hinder margin of 

 the segment next in front, and serves as a fulcrum 

 in the flexion and extension of the abdomen : it 

 also prevents 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, 

 and of two many-jointed filaments borne on its 

 distal end. These are 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. The thoracic portion of the 

 carapace arises as an outgrowth from the hinder 

 border of the head, which grows back over the 

 thoracic segments, and is continuous with them 

 dorsally. 

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



