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 branchiostegites; the number of gills 
Fic. 166.—Gecarcinus 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 scaphognathite, 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- 
