orifices ("regions pterygostomiennes"), appears in our animals to be
divided into small square compartments. Milne-Edwards has already
pointed this out as a particularly remarkable peculiarity. This
appearance is caused partly by small wart-like elevations, and partly
and especially by curious geniculated hairs, which to a certain extent
constitute a fine net or hair-sieve extended immediately over the
surface of the carapace. Thus when a wave of water escapes from the
branchial cavity, it immediately becomes diffused in this network of
hairs and then again conveyed back to the branchial cavity by vigorous
movements of the appendage of the outer maxilliped which works in the
entrant fissure. Whilst the water glides in this way over the carapace
in the form of a thin film, it will again saturate itself with oxygen,
and may then serve afresh for the purposes of respiration. In order to
complete this arrangement the outer maxillipeds, as indeed has long been
known, bear a projecting ridge furnished with a dense fringe of hairs,
which commences in front near their median line and passes backwards and
outwards to the hinder angle of the buccal frame. 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