PODOPHTHALMIA. 47 



excellent translation by Mr. Dallas of " Flir Darwin," and using to a large extent the 

 exact words, though that fact is not indicated by quotation marks. 



Among the numerous facts in the natural history of Crustacea, there is one which 

 appears of particular importance, namely, the character of the branchial cavity in the 

 air-breathing crabs. In the frog-crab {Ranina) of the Indian Ocean, which, accord- 

 ing to Rumphius, loves to climb up on the roofs of the houses, the anterior entrant 

 orifice to the branchial cavity is entirely wanting, according to Milne Edwards, and 

 the entrance of a canal, opening into the hindmost parts of the branchial cavity, is 

 situated beneath the commencement of the abdomen. The case is most simple in some 

 of the Grapsoidea, as in Aratus pisonii, a charming, lively crab, which ascends the 

 mangrove bushes 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 one of these animals was sitting 

 on my hand, I noticed that it elevated the hinder portion of its carapax, and that, by 

 this means, a wide fissure was opened upon each side, above the last pair of feet, 

 through which I could look far into the branchial cavity. 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 [Southern Brazil]. 

 Whilst the hinder part of the carapax 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 carapax never takes place. The animal, 

 therefore, opens its branchial cavity in front or behind, accordingly as it has to breathe 

 water or air. 



I have also observed the same elevation of the carapax in some species of the allied 

 genera, Sesarma and Cydograpsus, which dig deep holes in marshy ground, and often 

 run about in the wet mud, or sit, as if keeping watch, before their burrows. One 

 must, however, wait for a long time with these animals, when taken out of the water, 

 before they open their branchiar cavity to the air, for they possess a wonderful arrange- 

 ment, by means of which they can continue to breathe water for some time when taken 

 from that element. The orifices for the egress of the water which has served for res- 

 piration are situated in these, as in most crabs, in the anterior angle of the buccal 

 frame, while the entrant fissures of the branchial cavity extend from its hinder angles 

 above the first pair of feet. Now, that portion of the carapax which extends at the 

 sides of the mouth between the two orifices appears in our animals to be divided into 

 small, square compartments. This appearance is caused partly by small wart-like 

 elevations, and partly and especially by curious geniculated hairs, which, to a certain 

 extent, constitute a fine net or hair-sieve extended immediately over the surface of the 

 carapax. Thus, when a wave of water escapes from the branchial cavity, it imme- 

 diately bedomes diffused in this network, and then is again conveyed back to the 

 branchial cavity by vigorous movements of the appendage of the outer maxilliped, 

 which works in the entrant fissure. While the water glides in this way over the cara- 

 pax, in the form of a thin film, it will again saturate itself with oxygen, and may then 

 serve afresh for the purposes of respiration. In very moist air the store of water con- 

 tained in the branchial cavity may hold out for hours, and it is only when this is used 

 up that the animal elevates its carapax in order to allow the air to have access to its 

 branchisB from behind. 



In the Sand Crabs {Ocypoda) a peculiar arrangement on the third and fourth pairs 

 of feet has long been known, although its connection with the branchial cavity has not 



