The blood enters the ventral blood-chamber and spreads out through the entire cavity.
In the distal part of the gill where the partition separating the ventral from the dorsal chamber
comes to an end the blood passes into the dorsal blood-chamber and immediately into that portion
of it forming the main excurrent channel. The blood entering the ventral chamber of the special
part of the gill flows through the compartments of that chamber to the places where they communicate
with the corresponding compartments of the dorsal chamber and there enters the latter. It
flows through these to the main excurrent channel, excepting a small portion which passes into
the excurrent channel at the basal joint.
The fu n c tio n o f th e b lo o d -ch am b e r s . The function of the ventral chamber is obviously
to receive venous blood from the body and distribute it over the entire outer face of the
gill. It is there separated from the external atmosphere by its own wall and the ventral wall of
the gill. As this wall of the gill is rather thin it would appear that an exchange of gases may
take place between the blood in the ventral chamber and the external air. (This statement is intended
to apply here only to the general part of the gill; the exchange of gases in the special part
is considered below.) The function of the dorsal chamber is evidently to expose the blood still
further to the action of air (as described below) and finally to collect the purified blood into a
channel through which it passes back' to the body. The use of the middle chamber appears to be
passive rather than active. The space taken up, by it reduces, by .so much the depth of the
chambers above and below and thus secures the exposure of a very thin stream of blood to the
action of the air in the air-chambers.
Th e fu n c tio n of th e a ir -ch am b e r s . When the blood enters the ventral chamber of
the special part of the gill it is separated from the air in the corresponding air chamber only by
the single thin membrane which forms the outer boundary wall of the blood-chamber. The ordinary
conditions of the respiration of air are thus satisfied. As the blood passes at the margin of the
gill into the dorsal chamber it is further exposed to the action of air in the dorsal air-chamber
and the respiratory process is here completed.
The air-chambers, as already noted, do not communicate by openings with the exterior of
the gill. The contained air must therefore enter by penetration through the wall of the gill. The
wall, as we have seen, is here very thin, the hypoderm, as a rule, appearing to be withdrawn and
leaving only a very thin layer of chitine.
As has already been stated, when an animal is placed in water the appearance of air in
the gills passes away. This observation led to a number of experiments the chief purpose of which
was to gain evidence, in addition to the evidence derived from direct inspection and from the study
of the structure of the gill, that air is normally present in the chambers.1
1. An animal was placed in water until the appearance of air in the gills had passed away.
It was then killed and the tissues fixed by hot 83°/o alcohol. In sections of the gills prepared
from this specimen the air-chambers did not appear empty (as usual) but contained blood without
corpuscles, that is, blood plasma (appearing as granulated matter).
This experiment having indicated that under the conditions imposed the air was replaced,
in part, at least by plasma of blood, we next sought to modify the experiment in order to ascertain
whether the presence of blood in the chambers was due entirely to the conditions being artificial.
T h e s e exp e rim en ts were sugge sted to me b y D r. z u b St b a s s e n .
2. The preceding experiment was repeated, excepting that the animal was placed in normal
salt solution instead of water. The result was that the chambers contained blood plasma, but perhaps
less in quantity than in the previous case.
In this case the two fluids were of approximately equal densities and, it may be inferred,
tended to replace the air in equal measure. Allowance must be made, however, for the greater
thinness of the wall bounding the blood-chamber than the outer chitinous wall.
3. The experiment was repeated using a concentrated salt solution. The result was that
the chambers appeared as empty, or nearly so, little blood plasma having entered them.
In this case it would appear that the more concentrated fluid replaced, in the main, the air.
In all three cases the results become intelligible on the supposition that the chambers contain
air. Thus these experiments warrant the inference that the chambers normally contain air
and not blood.
In considering more precisely the function of the air chambers we must first take account
of the fact that they do not serve to increase the area of respiratory surface. They are not, as
in the case of Porcellio and its congeners formed by an inward fold of the wall of the gill but are
simply spaces filled with air, lying between the parallel walls of the blood cavity and the gill. But
it is to be noted that a provision for exposure of a large surface of blood to air is made by the
expandet dimensions of the special part of the gill and by the presence of these parts in all five
pairs of the outer gills.
In studying the outer gills of Porcellio we reached the conclusion that the respiratory tree
is an adaptation having a two-fold purpose; namely, first, to provide for a large surface of exposure
of blood to air and, second, to secure the protection of the blood against dessiccation from exposure
to air in the process of respiration. Now, in Oniscus, the former of these purposes being provided
for-by the means just noted — an increase of the total area of the special respiratory surface —
it is left to the air-chambers to prevent a too great loss of water from the blood in the process
of respiration. One may easily conceive that if nothing intervened between the blood and the outside
atmospheric air but the two very thin membranes of the wall of the blood cavity and the chitinous
wall of the gill, the two being in contact, dessiccation would follow. Again, phylogenetic
considerations lead us to expect that the respiratory surfaces must be maintained in a moist state
as a condition of their functional action.
In the respiratory process the air of the chambers becomes charged with water of respiration.
The only means of escape of this water to the outer atmospheric air is by passage through the
chitinous wall. This process takes place slowly and secures the maintenance of the moist condition
of the air in the chambers. This air is thus separated from the blood by only a very thin
moist membrane. Thus conditions most favorable to respiration are provided; the blood is both
shielded from dessiccation and at the same time separated from air by only a very slight barrier.
I reach the conclusion that through the possession of these special modifications, the outer
gills of Onisciis constitute organs for breathing ordinary dry atmospheric air. The several collateral
reasons in support of this conclusion that have already been given for Porcellio. apply also for Oniscus.
The In n e r Q-ills. The inner gills of Oniscus agree in all essential features with the
corresponding parts in Porcellio and the other genera already described.