194 



CRUSTACEA EUCARIDA DECAPODA 



FIG. 134. Dorsal view of Pachygrapsus mamoratus, 

 x ^. (From an original drawing prepared for Professor 

 Weldon.) 



the place of our common Carcinus maenas, which is not found 



there. 



Among the 

 land genera are 

 Ocypoda, Gelasi- 

 mus, and Gecar- 

 cinus of tropical 

 lagoons and coastal 

 swamps. Ocypoda 

 often occurs in 

 vast crowds in 

 these regions, and 

 digs burrows in 

 the sand. 



Gelasimus (Fig. 

 1 3 5 ) is remarkable 

 for the enormous 



size of one of the chelipedes. generally the right, in the male, 



which may actually exceed in size the rest 



of the body. It is not known for what 



purpose this organ serves in the various 



species. . In Gelasimus it is supposed that 



the male stops up the mouth of the burrow 



with it when he and the female are safely 



inside. It is also used as a weapon in sexual 



combats with other males ; but Alcock, from 



observations made in the Indian Ocean, be- 

 lieves that the males use it for exciting the 



admiration of the females in courtship, as 



the huge chela is bright red in colour, 



and the males brandish it about before the 



females as if displaying its florid beauty. 

 The species of Ocypoda are exclusively 



terrestrial, and cannot live for a day in 



water. The gills have entirely disappeared, 



J ' FIG. 135. Gelasimus 



and the branchial chambers are converted ammlipes, x i. A, 



into air-breathing lungs with highly vascular fAft^Vic cM mal6 ' 

 walls, the entrances into which are situated 



as round holes between the bases of the third and fourth pairs 

 of walking legs. As their name implies, they can run with 



