CIIAP. V. RESPIRATION IN LAND CRABS. 35 



but arise why such an arrangement for the diminution 

 of friction should be necessary in these particular Crabs 

 and between these two feet, leaving out of consideration 

 the fact that the remarkable brushes of hair, which on 

 the other hand must increase friction, also remain un- 

 explained. But as I was bending the feet of a large 

 Sand-Crab to and fro in various directions, in order to 

 see in what movements of the animal friction occurred 

 at the place indicated, and whether these might, per- 

 haps, be movements of particular importance to it and 

 such as would frequently recur, I noticed, when I had 

 stretched the feet widely apart, in the hollow between 

 them a round orifice of considerable size, through which 

 air could easily be blown into the branchial cavity, and 

 a fine rod might even be introduced into it. The 

 orifice opens into the branchial cavity behind a conical 

 lobe, which stands above the third foot in place of a 

 branchia which is wanting in Ocypoda. It is bounded 

 laterally by ridges, which rise above the articulation of 

 the foot, and to which the lower margin of the carapace 

 is applied. Exteriorly, also, it is overarched by these 

 ridges with the exception of a narrow fissure. This 

 fissure is overlaid by the carapace, which exactly at this 

 part projects further downwards than elsewhere, and in 

 this way a complete tube is formed. Whilst in Grapsus 

 the water is allowed to reach the branchiae only from 

 the front, I saw it in Ocypoda flow in also through the 

 orifice just described. 



In the position of posterior entrant orifice and the 

 accompanying peculiarities of the third and fourth 



D 2 



