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 branchiae only from the front, I saw
it in Ocypoda flow in also through the orifice just described.
In the position of posterior entrant orifice and the accompanying
peculiarities of the third and fourth pairs of feet, two other
non-aquatic species of the same family, which I have had the opportunity
of examining, agree with Ocypoda. One of these, perhaps Gelasimus
vocans, which lives in the mangrove swamps, and likes to furnish the
mouth of its burrow with a thick, cylindrical chimney of several inches
in height, has the brushes on the basal joints of the feet in question
composed of ordinary hairs. The other, a smaller Gelasimus, not
described in Milne-Edwards' 'Natural History of Crustacea,' which
prefers drier places and is not afraid to run about on the burning sand
under the vertical rays of the noonday sun in December, but can also
endure being in water at least for several weeks, resembles Ocypoda in
having these brushes composed of non-setiform, delicate hairs, indeed
even more delicate and more regularly constructed than in Ocypoda.
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