EUPAGITRUS. 499 



EXTERNAL CHARACTERS (PL I). 



In the segmentation of the body the Hermit Crabs 

 exhibit a wider divergence from what might be assumed 

 to be a typical Higher Crustacean condition than any of 

 their allied Decapods. There is no portion of the body 

 from which a segment could be taken which has not lost 

 its primitive design in its specialisation. In the 

 Macrura and Brachyura the simplicity of the abdominal 

 region is preserved, even if the cephalothorax is 

 specialised, but in Eupagurus the condition of the 

 abdomen is such that it is a matter of some difficulty to 

 demonstrate any segmentation at all. An abdominal 

 segment of the Crayfish (Astacus) would be found to 

 possess three kinds of protective plates : — The dorsal 

 tergum, the ventral sternum, and the two lateral pleura. 

 An appendage is attached on each side to the outer end 

 of the sternum. The sternum is attached to its fellows 

 in front and behind by means of a flexible arthrodial 

 membrane, and to each flanking pleuron by a hard 

 membrane — the epimeron. No such typically developed 

 segment is to be found on the Hermit Crab. The only 

 fully calcified portions of the abdominal region are the 

 first and last segments, and neither of these is in any 

 way normal. The peduncle never at any stage bears 

 limbs and although a narrow sternum is present in the 

 larva, it vanishes altogether in the adult animal, while 

 the sternum and the pleura of the sixth segment are 

 always in a more or less membranous condition. 



Cephalothorax The exoskeleton of the Decapoda 



has been described so frequently that it will be 

 unnecessary to do more than refer briefly to points 

 peculiar to the type under consideration. The first 

 character which calls for mention in the cephalothorax 



