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 Eriphia gonagra the entrant orifices of the respiratory 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.
(FIGURE 12. Posterior entrance to the branchial cavity of Ocypoda
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.
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