rhombea, Fab., natural size. The carapace and the fourth foot of the
right side are removed.

FIGURE 13. Points of some of the hairs of the basal joints of the foot,
magnified 45 diam.)

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.* (* As this was not observed in the sea, but in glass
vessels containing sea-water, it might be supposed that the animals
become exhausted and die, not because they are under water but because
they have consumed all the oxygen which it contained. I therefore put
into the same water from which I had just taken an unconscious Ocypoda,
with its legs hanging loosely down, a specimen of Lupea diacantha which
had been reduced to the same state by being kept in the air, and this
recovered in the water just as the Ocypoda did in the air.) In these a
peculiar arrangement on the feet of the third and fourth pairs (Figure
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 (Figure 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