surface of exposure, and second, a thinning of the same wall, in order to secure the permeability
requisite for a ready exchange of elements between, the two media.
But modifications of structure in each of these two directions are subject to limitations
imposed by other mechanical conditions. Increase of surface is limited by the requirement of a
moderate and proportionate size of the part; and the thinning out of the wall is limited by the
requirement of strength.
Applying these principles we see that in the specially modified portion of the chitinous wall
at the exterior of the gill increase of surface and at the same time thiness of wall is secured through
the furrowing of the chitine. Furthermore, the furrowed structure is that which in the least degree
compatible with the attainment of these two ends detracts from the strength of the wall. The
arrangment of the furrows in a net-work is adapted to securing the largest linear extent of grooving
and at the same time the least sacrifice of strength of the wall. The shape of the furrows which,
as seen in section, (Fig. 8 a,- gr.) is that of an oval with the long axis at right angles to the face
of the wall is, I conceive, adapted to a three-fold purpose, namely: first, to containing a large
amount of air relative to the space occupied; second, to retaining this air in the groove (by means
of the narrowing of the opening); and third, to reduce to the least extent the strength of the wall
(by means of the resistance to fracture secured through the curved surface of the oval).
In spite of these adaptations, it is evident that the grooved portion of the wall of chitine
has not the same strength as the rest of the wall. As a compensation for this the marginal region
of the grooved area is depressed below the level of the plain area and is bounded by a ridge of
chitine (ltd. Fig. 4) formed by a thickening of the wall at the line of junction of the two areas.
This ridge is the curved line, convex toward the middle of the gill, mentioned above in
the description of the appearance of the inner face of the gill. Furthermore, the grooved area as
a whole is bounded by convex surfaces — a means of increasing its strength.
In the tree extent of surface is secured by the folding of the wall and permeability by the
thinning out of the same. The protected position of the tree inside the gill permitts a high degree
of tenuousness of the wall. The tubular form of the branches, as well as their rounded form at
the ends, afford the greatest degree of strength consistent with their extreme thinness.
The tree, as a mass, is somenhat spherical in shape (Fig. 1), which secures the most favorable
distribution of the branches in the fluid surrounding them. But the sphere is depressed on
the basal side of the tree, leaving a space between the base and the grooved area of wall of the
gill, as noted above (p. 22, see also Fig. 4, B. ch.). It is evident that this is an adaptation for
bringing a relatively large amount of blood into relation with the air contained both in the grooves
and in the basal branches of the tree.
The c ir cu la tio n of th e b lo o d in th e g ill. From my observations I am lead to infer
that all of the blood in the course of its circulation through the gill passes through the space or
chamber, just referred to. By mounting a livipg animal in water the movements of the blood corpuscles
within the gill can in part be observed. The blood enters the gill at its connection with
the basal joint and is first distributed through the cavity of the general part of the gill. Due to
the position of the gill in relation to the joint the course of the current is directed toward the
inner and lower sides of the blood cavity. But owing to the curvature of the inner side of the
cavity (corresponding to the curved outline of the side of the gill lying toward the middle line of
the body) the blood particles are swerved in their course toward the special part of the gill. They
may be seen in part to enter among the branches of the tree and in part to enter the large space
above described, at its posterior end. I have not found it possible to trace further the course of
the circulation but it would appear that the blood, entering among the branches of the tree, passes
through the tree into the cavity, there mingling with that which entered at the posterior end of the
cavity. The current would then pass upwards and outwards toward the excurrent channel at the
base of the gill.
The comparatively large size of this special blood space causes a slower movement of the
blood through the space and thus permits a longer exposure to the air contained in the grooves
and in the basal branches of the tree.
The c om p o sitio n o f th e air in th e tr e e . I have thus far spoken of the contents of
the tree and the grooves as consisting of air. It is to be understood, however, that it is air altered
in composition through the respiratory process. And inasmuch as there is no mechanism for renewing
the supply of air, this alteration must be carried to a considerable degree. In general it may
be considered that its composition is that of ordinary air to which has been added in the respira-
tory process a quantity of carbonic acid from the blood and from which has been taken a corresponding
quantity of oxygen.
We have next to consider the condition of the air in the tree in respect to presence of
moisture. Concerning this point D u v e r n o y and L e r e b o u l l e t in their joint work went so far as
to state that the white body absorbs the moisture of the air. L e r e b o u l l e t in his later work
reached the conclusion that the body contains air but that it is indispensable for the functional
action of the gill that the air taken up from without should be charged with moisture. Other
authors have made no express statements upon this point.
In considering this question we are first met by the fact that the animals live in situations
where the air is damper than ordinary atmospheric air. One may suppose, however, that this is in
adaptation to the functional action of the inner gills only. Next, there are phylogenetic considerations
which appear rather adverse to the view that the gills are adequate to breathing ordinary air. The
ancestral Isopoda were aquatic animals and their descendants comprising the modern family of the
Oniscidae have acquired the terrestrial habit of life by a gradual process. It would appear that
the modifications of the gills in adaptation to the respiration of air may not have been carried to
the extent that they are capable of breathing ordinary dry atmospheric air.
Notwithstanding these considerations I have reached the conclusion that the outer gills of
Porcellio (and its congeners) are capable of functioning in a medium of atmospheric air in its ordinary
condition as to quantity of moisture present. First, on the basis of the structure of the gill
it would appear that there is warrant for the inference. Since the tree corresponds in structural
principle with the tracheae of insects which in general live under ordinary conditions as to atmosphere,
it would seem probable that it is capable of the same functional action. Moreover, I conceive
that the form and situation of the respiratory tree are adaptations to this end. The tree,
indeed, in its general build and relations possesses a two-fold adaptation. The first we have considered
above, namely, the bringing of air into relation with blood. The second is to secure the
protection of the blood, against dessiccation from the air in the process of respiration. We have
seen that the mass of air present in the tree is changed very slowly, due to the shope of the tree
(having only a single orifice) and the lack of any mechanism for inspiration and expiration. This
secures also a retardation in the escape of the-water of respiration — that passing off from the
Bibliotheca zoologie». Heft 25. 3