17 
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 hut 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 composition of the air in the tree. 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 renew- 
ing the su])ply of air, this alteration must he carried to a consideral)le 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 corre- 
sponding 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 Duvernoy and Lerekoullet in their joint work went so far as 
to state that the white body absorbs the moisture of the air. Lereboullet in his later work 
reached the conclusion that the body contains air hut that it is indispensable for the functional 
action of the gill that the air taken uj) fi'om without should he 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 siqipose, 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 liabit 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 caiTied 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 or- 
dinary 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 atmo- 
sphere, it would seem probable that it is capable of the same functional action. Moreover, I con- 
ceive 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 con- 
sidered 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 jjassing oft’ from the 
Bibliotheca zoologica. Heft 25. 3 
