126 GEOGRAPHICAL DISTRIBUTION. 



retiring to the neighbouring plains, or woods, 

 they repose for some time, and then the females 

 return to the water, and commit their eggs to the 

 waves. This business dispatched, they endea- 

 vour to regain, in the same order, the country 

 they had left, and by the same route, but only 

 the most vigorous can reach the mountains. The 

 greater part are so weak and lean, that they are 

 forced to stop to recruit their strength in the first 

 country they reach. When arrived again at their 

 habitations, they have a new labour to undergo, 

 for now is the time of their moult. They hide 

 themselves in their subterranean retreats for this 

 purpose, so that not a single one can be seen : 

 they even stop up the mouth of their burrows. 

 Some writers, however, affirm that they change 

 their shells immediately after their oviposition. 



The respiration of these land- crabs, for a long 

 time, had puzzled comparative anatomists. 

 They could not explain how animals, breathing 

 by gills, could subsist so long out of the water 

 without these organs becoming useless. M. M. 

 Audouin, however, and Milne Edwards, cleared 

 up the mystery by the discovery of a kind of 

 trough, formed by the folds which line and con- 

 stitute the parietes of the branchial cavity, and 

 destined to contain and preserve a certain quan- 

 tity of water proper to moisten the gills. One 

 species 1 has more than one pocket, or vesicle, 



1 Gecarcinus Uca. 



