Fig. 12. Posterior entrance to the branchial cavity of Ocypoda rhombea, Fab. The carapace and the fourth foot of the right side are removed. Fig. 13. Points of some of the hairs of the basal joints of the foot, magnified. Fig. 12. Posterior entrance to the branchial cavity of _Ocypoda rhombea,_ Fab. The carapace and the fourth foot of the right side are removed. Fig. 13. Points of some of the hairs of the basal joints of the foot, magnified. The swift-footed Sand-Crabs (_Ocypoda_) are exclusively 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.[1] In these a peculiar arrangement on the feet of the third and fourth pairs (Fig. 12) has long been known, although its connexion with the branchial cavity has not been suspected. These two pairs of feet are more closely approximated than the rest; the opposed surfaces of their basal joints (therefore the hinder surface on the third, and the anterior surface on the fourth feet) are smooth and polished, and their margins bear a dense border of long, silky, and peculiarly formed hairs (Fig. 13). Milne-Edwards who rightly compares these surfaces, as to their appearance, with articular surfaces, thinks that they serve to diminish the friction between the two feet. In considering this interpretation, the question could not 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 unexplained. 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, perhaps, 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 branchiæ only from the front, I saw it in _Ocypoda_ flow in also through the orifice just described.