RESPIRATION IN LAND CRABS.
Among the numerous facts in the natural history of the Crustacea upon
which a new and clear light is thrown by Darwin’s theory, besides the
two forms of the males in our _Tanais_ and in _Orchestia Darwinii,_
there is one which appears to me of particular importance, namely, the
character of the branchial cavity in the air-breathing Crabs, of which,
unfortunately, I have been unable to investigate some of the most
remarkable (_Gecarcinus, Ranina_). As this character, namely, the
existence of an entrance behind the branchiæ, has hitherto been
noticed, even as a fact, only in _Ranina,_ I will go into it in some
detail. 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 Grapsoidæ, 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. Whilst the hinder part of
the carapace rises and the above-mentioned fissure is formed, the
anterior part seems to sink, and to narrow or entirely close the
anterior entrant orifice. Under water the elevation of the carapace
never takes place. The animal therefore opens its branchial cavity in
front or behind, according as it has to breathe water or air. How the
elevation of the carapace is effected I do not know, but I believe that
a membranous sac, which extends from the body cavity far into the
branchial cavity beneath the hinder part of the carapace, is inflated
by the impulsion of the fluids of the body, and the carapace is thereby
raised.
I have also observed the same elevation of the carapace in some species
of the allied genera _Sesarma_ and _ Cyclograpsus,_ which dig deep
holes in marshy ground, and often run about upon the wet mud, or sit,
as if keeping watch, before their burrows. One must, however, wait for
a long time with these animals, when taken out of the water, before
they open their branchial cavity to the air, for they possess a
wonderful arrangement, by means of which they can continue to breathe
water for some time when out of the water. The orifices for the egress
of the water which has served for respiration, are situated in these,
as in most Crabs, in the anterior angles of the buccal frame (“cadre
buccal,” M.-Edw.), whilst the entrant fissures of the branchial cavity
extend from its hinder angles above the first pair of feet. Now that
portion of the carapace which extends at the sides of the mouth between
the two orifices (“régions ptérygostomiennes”), 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 branchiæ from behind.
In _Eriphia gonagra_ the entrant orifices of the respiratory cavity
serving for aerial respiration are situated, not, as in the Grapsoidæ,
above, but behind the last pair of feet at the sides of the abdomen.
Fig. 12. Posterior entrance to the branchial cavity of Ocypoda rhombea,
Fab. The carapace and the fourth foot of the right side are removed.
Fig. 13. Points of some of the hairs of the basal joints of the foot,
magnified. Fig. 12. Posterior entrance to the branchial cavity of
_Ocypoda rhombea,_ Fab. The carapace and the fourth foot of the right
side are removed. Fig. 13. Points of some of the hairs of the basal
joints of the foot, magnified.
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.[1] In these a peculiar arrangement on the feet of the
third and fourth pairs (Fig. 12) has long been known, although its
connexion with the branchial cavity has not been suspected. These two
pairs of feet are more closely approximated than the rest; the opposed
surfaces of their basal joints (therefore the hinder surface on the
third, and the anterior surface on the fourth feet) are smooth and
polished, and their margins bear a dense border of long, silky, and
peculiarly formed hairs (Fig. 13). Milne-Edwards who rightly compares
these surfaces, as to their appearance, with articular surfaces, thinks
that they serve to diminish the friction between the two feet. In
considering this interpretation, the question could not but arise why
such an arrangement for the diminution of friction should be necessary
in these particular Crabs and between these two feet, leaving out of
consideration the fact that the remarkable brushes of hair, which on
the other hand must increase friction, also remain unexplained. But as
I was bending the feet of a large Sand-Crab to and fro in various
directions, in order to see in what movements of the animal friction
occurred at the place indicated, and whether these might, perhaps, be
movements of particular importance to it and such as would frequently
recur, I noticed, when I had stretched the feet widely apart, in the
hollow between them a round orifice of considerable size, through which
air could easily be blown into the branchial cavity, and a fine rod
might even be introduced into it. 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 branchiæ 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._[2] 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
surprising. If, in the nearly allied families of the Ocypodidæ and
Grapsoidæ, the closest agreement prevails in all the essential
conditions of their structure; if the same plan of structure is
slavishly followed in everything else, in the organs of sense, in the
articulation of the limbs, in every trabecula and tuft of hairs in the
complicated framework of the stomach, and in all the arrangements
subserving aquatic respiration, even to the hairs of the flagella
employed in cleaning the branchiæ,—why have we suddenly this exception,
this complete difference, in connection with aerial respiration?
The schoolmen will scarcely have an answer for this question, except by
placing themselves on the theologico-teleological stand-point which has
justly fallen into disfavour amongst us, and from which the mode of
production of an arrangement is supposed to be explained, if its
“adaptation” to the animal can be demonstrated. From this point of view
we might certainly say that a widely gaping fissure which had nothing
prejudicial in it to _ Aratus Pisonii_ among the foliage of the
mangrove bushes, was not suitable to the _Ocypoda_ living in sand; that
in the latter, in order to prevent the penetration of the sand, the
orifice of the branchial cavity must be placed at its lowest part,
directed downwards, and concealed between broad surfaces fringed with
protective brushes of hair. It is far from the intention of these pages
to enter upon a general refutation of this theory of adaptation. Indeed
there is scarcely anything essential to be added to the many admirable
remarks that have been made upon this subject since the time of
Spinoza. But this may be remarked, that I regard it as one of the most
important services of the Darwinian theory that it has deprived those
considerations of usefulness which are still undeniable in the domain
of life, of their mystical supremacy. In the case before us it is
sufficient to refer to the Gelasimus of the mangrove swamps, which
shares the same conditions of life with various Grapsoidæ and yet does
not agree with them, but with the arenicolous _Ocypoda._
[1] 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. I therefore put into
the same water from which I had just taken an unconscious _Ocypoda,_
with its legs hanging loosely down, a specimen of _Lupea diacantha_
which had been reduced to the same state by being kept in the air, and
this recovered in the water just as the _Ocypoda_ did in the air.
[2] 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.