232 MANUAL OF ZOOLOGY. 



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. 75) are mostly 

 furnished with ambulatory limbs, and are rairely formed for 

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

 even living inland. 



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 {Gecarcinus) 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 

 countries of the Old and New Worlds, as well as Australia. 

 They are essentially terrestrial in 

 their habits, and migrate in large 

 bodies to the sea, in order to lay 

 their eggs. Besides the true Gecar- 

 cini, members of other very different 

 families live more or less constantly 

 on dry land, and have air admitted 

 directly into the branchial chamber. 

 Amongst these are the Calling-crabs 

 (Gelasintus), the Frog-crabs (Rani- 

 nd), and the Sand-crabs {Ocypoda). 



Reproduction in the Crabs is the 

 same as in the Macrura, but the 

 Fig. 76.— Larva (Zoea) of Crab larva is exceedingly unlike the adult, 

 fcto KiS"'"^' ""'^"'' and approximates closely to the type 

 of the Macrwa, another proof that 

 the Brachyura stand higher in the Crustacean scale. The 

 larval Crab was originally described as a distinct animal, under 

 the name of Zoea (fig. 76), presenting in this condition a long 

 and well-developed abdomen. It is only after several succes- 

 sive moults that the young Ci*ab assumes its characteristic 

 Brachyurous form, and acquires by gradual changes the features 

 which distinguish the adult. The Zoece of the Crabs are usually 

 distinguished by the possession of long spines developed from 

 the carapace. When first liberated from the egg, the Zoea is 

 enveloped in a larval skin or membrane, which is shed in a 

 few hours. 



