CHAP. V. RESPIRATION IN LAND CRABS. 31 



The case is most simple in some of the Grapsoidae, 

 as in Aratus Pisonii, a charming, lively Crab which 

 ascends the mangrove bushes (Rhizophora) and gnaws 

 their leaves. By means of its short but remarkably 

 acute claws, which prick like pins when it runs over the 

 hand, this Crab climbs with the greatest agility upon 

 the thinnest twigs. Once, when I had one of these 

 animals sitting upon my hand, I noticed that it elevated 

 the hinder part of its carapace, and that by this means 

 a wide fissure was opened upon each side above the lat 

 pair of feet, through which I could look far into the 

 branchial cavity. I have since been unable to pro- 

 cure this remarkable animal again, but on the other 

 hand, I have frequently repeated the same observation 

 upon another animal of the same family (apparently a 

 true Grapsus), which lives abundantly upon the rocks 

 of our coast. Whilst the hinder part of the carapace 

 rises and the above-mentioned fissure is formed, the 

 anterior part seems to sink, and to narrow or entirely 

 close the anterior entrant orifice. Under water the 

 elevation of the carapace never takes place. The 

 animal therefore opens its branchial cavity in front or 

 behind, according as it has to breathe water or air. 

 How the elevation of the carapace is effected I do not 

 know, but I believe that a membranous sac, which 

 extends from the body cavity far into the branchial 

 cavity beneath the hinder part of the carapace, is in- 

 flated by the impulsion of the fluids of the body, and 

 the carapace is thereby raised. 



