ARTHROPODA 287 



This group includes the crabs, which are classified in a 

 great number of families. 



In Decapods the region of the head is usually marked off 

 from the thoracic portion by a fold in the carapace, the cervical 

 groove ; and in the larger forms the cephalothoracic shield is 

 frequently divided up into areas corresponding to the position 

 of various internal organs. The last thoracic segment occasion- 

 ally remains moveable on the others. 



The sides of the carapace enclose the branchial chamber, 

 and are known as IrraTichiostegUes ; the number of gills 



FlQ. 166. — Geca/rcinus ruricola (land-crab of Monserral, West Indies). 



varies. The water-supply enters through the slit-like opening 

 between the edge of the branchiostegite and the body in the 

 Macrura, but in the Brachyura there is a special narrow opening 

 situated in front of the first pair of walking-legs.' The current 

 is maintained by a specialised portion of the second maxilla, 

 the scwphognathite, which flaps to and fro. This process either 

 represents the epipodite, or the epipodite and exopodite fused. 

 The two pairs of maxillae to some extent retain the foliaceous 

 character of the primitive Phyllopod appendage, but all the 

 other appendages depart widely from this type. 



A few Decapods, as Birgus latro, allied to the hermit- 



