19 



a groove, but is continuous with the sternum of the same 

 segment. 



In their natural position the gills lie upon the 

 thoracic epimera. 



THE ABDOMEN. 



The abdomen is continuous with the posterior part 

 of the cephalothorax. The connection is effected by means 

 of an arthrodial membrane, which allows of considerable 

 movement between the two regions. The abdomen is 

 small and in its natural position is closely applied to the 

 sternal region of the thorax. This region differs in the 

 two sexes, being much broader in the female than in the 

 male. This character provides a useful and ready method 

 of distinguishing between the two sexes. There are other 



differences which require a more detailed examination. 







Female. 

 (PI. I, fig 2, PI. IV, fig. 32, PI. V, fig. 34.^ 



This consists' of six somites and the telson, all of 

 which are freely movable. When lying in position it 

 extends as far forward as the posterior end of the sternum 

 of the third thoracic somite. The locking arrangement 

 for keeping the abdomen closely applied to the thoracic 

 sternum is not so well developed as in the male. It 

 consists of two extremely small tubercles on the fifth 

 thoracic sternum which fit into two slight depressions at 

 the postero-lateral corners of the ventral side of the sixth 

 abdominal somite. The total length of the abdomen is 

 2-| times as much as its greatest width. 



There are four pairs of appendages, one pair being 

 borne on the second and on each of the three following 

 somites, respectively. 



