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.* (*
This smaller Gelasimus is also remarkable because the chameleon-like
change of colour exhibited by many Crabs occurs very strikingly in it.
The carapace of a male which I have now before me shone with a dazzling
white in its hinder parts five minutes since when I captured it, at
present it shows a dull gray tint at the same place.) What may be the
significance of these peculiar hairs,--whether they only keep foreign
bodies from the branchial cavity,--whether they furnish moisture to the
air flowing past them,--or whether, as their aspect, especially in the
small Gelasimus, reminds one of the olfactory filaments of the Crabs,
they may also perform similar functions,--are questions the due
discussion of which would lead us too far from our subject. Nevertheless
it may be remarked that in both species, especially in Ocypoda, the
olfactory filaments in their ordinary situation are very much reduced,
and when they are in the water their flagella never perform the peculiar
beating movements which may be observed in other Crabs, and even in the
larger Gelasimus; moreover, the organ of smell must probably be sought
in these air-breathing Crabs, as in the air-breathing Vertebrata, at the
entrance to the respiratory cavity.

So much for the facts with regard to the aerial respiration of the
Crabs. It has already been indicated why Darwin's theory requires that
when any peculiar arrangements exist for aerial respiration, these will
be differently constructed in different families. That experience is in
perfect accordance with this requirement is the more in favour of
Darwin, because the schoolmen far from being able to foresee or explain
such profound differences, must rather regard them as extremely