250 MANUAL OF ZOOLOGY. 



Crabs, are distinguished from the two preceding tribes by the 

 rudimentary condition of the abdomen, which is very short, 

 and is tucked up beneath the cephalothorax, the latter being 

 disproportionately large. The extremity of the abdomen is 

 not provided with any appendage, and it is merely employed 

 by the female to carry the ova. The Crabs (fig. 96) are mostly 

 furnished with ambulatory limbs, and are rarely formed for 

 swimming, most of them being littoral in their habits, and some 

 even living inland. 



Fig. 96. Brachyura. The Spiny Spider-Crab (Maia squinado). 



In all the essential points of their anatomy the Crabs do not 

 differ from the Lobster and the other Macrura ; but they are 

 decidedly higher in their organisation. This is especially seen 

 in the disposition of the nervous system, the ventral ganglia 

 in the Crab being concentrated into a single large ganglion, 

 from which nervous filaments are sent to all parts of the body. 

 In the Land-crabs (Gecardnus) respiration is by branchiae, 

 but there is almost always an aperture behind the carapace for 

 the admission of air. They are distributed over the warm 



