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
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.
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