ANNULOSA: CRUSTACEA. 



well developed, one being always larger than the other. Other 

 forms of the Anomura are the Sponge-crabs (Dromia), the Crab- 

 lobsters (Parcel lance), and the Tree-crabs (Birgus). 



TRIBE C. BRACHYURA. The "short-tailed" Decapods, or 

 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. 159) 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. 



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 espe- 

 cially 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 (Gecar- 

 tinus} respiration is by 

 branchiae, but there is al- 

 most 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 essen- 

 tially terrestrial in their ha- 

 bits, and migrate in large 

 bodies to the sea, in order 

 to lay their eggs. Besides 

 the true Gecarcini^ members 

 . of other very different fam- 

 ilies live more or less con- 

 stantly on dry land, and 

 have air admitted directly into the branchial chamber. Amongst 

 these are the Calling crabs (Gelasimus) and the Sand-crabs 

 ( Ocypoda). 



Fig. 1 60. Zoea of the Spiny Spider-Crab 

 (Maia squinado), enlarged. 



