blood to the outer air. It results that the water of respiration becomes distributed through the
mass of air in the tree and thus prevents a dessiccating action of the air upon the blood. Thus
while it might happen that if ordinary dry atmospheric air were brought into relation with the
respiratory surface of the blood cavity it would be fatal to the respiratory process, yet it remains
time that the outer gills are capable of functioning when the animals are surroundet by the ordinary
atmospheric medium.
As supporting this view we have the fact that the animals will live for a considerable length
of time under ordinary atmospheric conditions. I have found that Porcellio scaber lives from 24 to
36 hours when kept in air under the conditions of an ordinary living-room. I infer that under
these circumstances the breathing process takes place mainly in the outer gills and that finally death
ensues through the dessiccation of the inner gills only. (See also below under physiology of the
inner gills.)
C o n c lu s io n s . From the foregoing facts and considerations I reach the following conclusions
in regai'd to the nature of the outer branches of the first and second pairs of abdominal
appendages in Porcellio scaber.
1. From the stand-point of morphology they are gills, homologous with the true gills of
aquatic isopods.
2. But from the stand-point of physiology they are organs for the respiration of air.
3. The special respiratory organ forming the tree is in morphological principle homologous
with the tracheae of insects but it is différent in structural plan, especially as having only a single
external opening and as lacking spiral folds of the chitinous wall.
4. In its general physiological value the tree corresponds to tracheae, but it differs from
tracheae in the respects first,-that it is adapted to retaining for a time the water of respiration in
the air-cavity and second, that it is an organ lying in a particular region of the body to which the
blood is brought, to be aerated, instead of a system of tubes penetrating the body in order to carry
air to the blood.
5. In the respect last mentioned the tree is analogous to the lungs of vertebrate animals,
but
6. Morphologically thè tree differs from lungs, first, in that it lacks any associated miiscular
mechanism for forcing air in and out and, second, it is an infold of the external wall of the body,
instead of the wall of the enteric cavity.
The la s t three pairs of outer gills. These lack the respiratory tree and the area of
grooved chitinous wall. That is to say, they have no special modifications adapting them to the
respiration of air. In other respects they correspond in structure to the first and second pairs.
As regards their physiological office it is evident from their form and position that they
serve as covers for the corresponding three pairs of inner gills. And since the inner gills can
perform their function only in air charged with moisture (see below) it would appear that the outer
gills exercise their protective function mainly in sheltering the inner gills from the dessiccation that
would follow from direct exposure to air.
The movements of the outer gills have already been referred to (p. 5). It is evident that
the object of the movements in to afford a renewal of the supply of air to the inner gills. In a
living specimen under a lens one can see the gills separate and a layer of air enter between each
lateral division and the one that lies nest posterior to it. As the gills come together again, this
— IS ■
layer of air appears to be pressed into the cavity in which the inner gills lie. The outer gills thus
possess an active as well as a passive function with respect to the inner gills.
It is possible that the outer gills, in addition to performing these functions, constitute in
themselves organs of respiration. The blood circulates between the two walls of the gills, and
whether respiration takes place depends upon- the penetrability of the walls to the gases to be exchanged.
The ventral wall of chitine is thick — thicker than the chitinous wall in some other
regions of the body, as, for example, the thoracic legs (in which also the blood circulates). The
dorsal wall is, on the other hand, moderately thin. It is, moreover, exposed to the moist air in
the chamber in which the inner gills lie. It would seem probable, therefore, that to some extent
respiration may take place through the dorsal wall.
I conclude, then, that the function of the last three pairs of outer gills is chiefly a relative
one, namely, that of protecting the inner gills from dessiccation and of supplying air to them; but
that they also act independently, as organs of respiration, though limited in this function.
The Inner Gills. The general external features of the inner gills of Porcellio have been
studied by several authors: first by Treviranus and later by Duvernoy, Lereboullet and Leydig,
The last named author also investigated their internal structure in the species Porcellio armadilloides.1
I find that the figure he gives for this species applies also in general to Porcellio scaber. (Full
notice of Leydig’s account of the histology will be given below under Gylisticus convexicus.)
A cross-section of the inner gill of Porcellio scaber presents the appearance shown in Fig. 8.
It is seen to consist of a flattened sac the cavity of which contains blood. The wall of the sack
is composed of an outer thin layer of chitine and an inner thick nucleated layer — the hypoderm.
The hypodermic layers on the two opposite dorsal and ventral sides of the gill are connected by
pillars. The lines representing the structural elements of these pillars are seen to be continuous
at each end with the structural elements of the hypodermic layer.
Examined as to its minute anatomy the hypoderm is seen to consist of a continuous layer
of granular and striated matter. In other words there are no cell-boundaries, the cell structure
being indicated alone by the nuclei. The striae of the protoplasm are arranged at right angles to
the surface of the gill. The nuclei are large and in general ovoidal in shape. They show a definite
arrangment, — the long axis of the ovoid being at right angles to the face of the gill and the
narrow end on the side toward the cavity of the gill. The granules of chromatine of the nuclei
also conform to a law of arrangment, namely-, they tend to be aggregated at the narrow end of
the nucleus, toward the cavity of the gill. It would appear that these structural features have a
significance in reference to the* passage of the gases concerned in respiration through the hypodermic
layer.T
he hypoderm at its inner margin is bounded by a clearly defined line which is immediately
in contact with the blood within. The appearance of this line is such as to suggest that it is
the dense (chitinized ?) inner surface layer of the hypodermic tissue. Whether this is the case or
whether the line represents a remnant of a mesodermic layer bounding the blood cavity, I have not
had an opportunity, through the study of tha. embryology of the gill, to ascertain.
P h y s io lo g y of th e In n e r G ills. The function of the inner gills is that of respiration
in a medium of moist air. That the animals require air charged with moisture as a condition for
Concerning th e id e n tity o f th is species, se e p. 20.