330 On the Variations of certain Crustacea. 



this would certainly furnish a strong argument in favour of 

 the theory, and the latter alternative has proved to be the 

 true one. 



In an Aratus which climbs upon the branches of the man- 

 groves;, and in a Grajosus which runs about the rocks of Santa 

 Catharina, the air finds entrance to the branchial cavity by a 

 fissure situated above the last pair of feet. These crabs open 

 the respiratory fissure by elevating the posterior extremity of 

 the carapace. This aperture is consequently at the extremity 

 of the branchial cavity opposite to that by which water enters 

 and issues ; for the apertures for the ingestion and egestion 

 of water are in the same position in all crabs. 



The genera Sesarma and Gyclograpsus, belonging, like the 

 preceding, to the family Grapsidoe, contain species living in 

 holes on the shore. These possess the same posterior res- 

 piratory fissure : but it is difficult to see the gaping, as the 

 animals rarely open it, indeed only when they have been a 

 long time out of the water. This is due to a very curious 

 arrangement, which does not exist in the preceding species, 

 and which enables these animals for a long time to respire the 

 air dissolved in the water that bathes the branchiae. The 

 region which separates the aperture for the reception and 

 emission of water is, as it were, reticulated, and bristles with 

 small recurved hairs. The water issuing from the egestive 

 orifice spreads in an instant over this network of hairs, and 

 becomes saturated with air, after which it is conducted by a 

 special arrangement into the ingestive aperture. The same 

 portion of water may thus pass through the branchial chamber 

 a great many times, carrying always a fresh supply of oxygen 

 with it. In moist air this circulation of water may be main- 

 tained for a very long time, but when the provision of water is 

 evaporated, the crab has recourse to the posterior aperture for 

 aerial respiration. 



The arenicolous Ocypodce have been so completely estranged 

 from an aquatic mode of life that a stay of one day in sea- 

 water is sufficient to kill them. It has long been observed 

 that in these animals the third and fourth pairs of feet arc 

 exceedingly close together. Their contiguous surfaces are 

 clothed at the margins with a dense coat of hairs. It has 

 been supposed that these hairs were intended to diminish the 

 friction of the surfaces, but this is evidently a mistake. Dr. 

 Muller has discovered between the bases of these approximated 

 legs an aperture leading into the respiratory cavity. This 

 arrangement exists in several species of the family, particularly 

 in certain Gelasimi, some of which inhabit the mangrove 

 swamps, whilst others run about upon the sand in open day. 

 It might, perhaps, be said that this arrangement was necessary 



