I have already mentioned that, as indeed is required by Darwin's
theory, this entrant orifice is produced in different manners in the
different families.
In the Frog-crab (Ranina) of the Indian Ocean, which, according to
Rumphius, loves to climb up on the roofs of the houses, the ordinary
anterior entrant orifice is entirely wanting according to Milne-Edwards,
and the entrance of a canal opening into the hindmost parts of the
branchial cavity is situated beneath the commencement of the abdomen.
The case is most simple in some of the Grapsoidae, as in Aratus Pisonii,
a charming, lively Crab which ascends the mangrove bushes (Rhizophora)
and gnaws their leaves. By means of its short but remarkably acute
claws, which prick like pins when it runs over the hand, this Crab
climbs with the greatest agility upon the thinnest twigs. Once, when I
had one of these animals sitting upon my hand, I noticed that it
elevated the hinder part of its carapace, and that by this means a wide
fissure was opened upon each side above the last pair of feet, through
which I could look far into the branchial cavity. I have since been
unable to procure this remarkable animal again, but on the other hand, I
have frequently repeated the same observation upon another animal of the
same family (apparently a true Grapsus), which lives abundantly upon the
rocks of our coast.
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