Chap. Y. EESPIRATION IN LAND CRABS. 33 



outer maxilliped which works in the entrant fissure. 

 Whilst the water glides in this way over the carapace 

 in the form of a thin film, it will again saturate itself 

 with oxygen, and may then serve afresh for the pur- 

 poses of respiration. In order to complete this arrange- 

 ment the outer maxillipeds, as indeed has long been 

 known, bear a projecting ridge furnished with a dense 

 fringe of hairs, which commences in front near their 

 median line and passes backwards and outwards to the 

 hinder angle of the buccal frame. Thus the two ridges 

 of the right and left sides form together a triangle with 

 the apex turned forwards, — a breakwater by which the 

 water flowing from the branchial cavity is kept away 

 from the mouth and reconducted to the branchial 

 cavity. In very moist air the store of water contained 

 in the branchial cavity may hold out for hours, and it 

 is only when this is used up that the animal elevates its 

 carapace in order to allow the air to have access to its 

 branchiae from behind. 



In Eri/phia gonagra the entrant orifices of the respi- 

 ratory cavity serving for aerial respiration are situated, 

 not, as in the Grapsoidae, above, but behind the last 

 pair of feet at the sides of the abdomen. 



The swift-footed Sand-Crabs (Ocijpoda) are exclu- 

 sively terrestrial animals, and can scarcely live for a 

 single day in water ; in a much shorter period a 

 state of complete relaxation occurs and all voluntary 

 movements cease.^ In these a peculiar arrangement 



1 As this was not observed in the sea, but iu glass vessels containing 

 sea-water, it might be supposed that the animals become exhausted 



D 



