— 17 — 



may be seen in part to enter among the branches of tbe 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 cii-culation but it woukl appear that the blood, entering among the branches of the tree, passes 

 through the tree into the cavity, there mingiing with that which entered at the posterior end of the 

 cavity. The ciirrent woukl 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 com Position 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 supply of air, tliis 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 (piantity of carbonic acid from the blood and from wliicli 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 Lekeuoullet in their Joint work went so far as 

 to State that the white body absorbs the moisture of the air. Lebeboullet in bis 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 l)e 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 giUs are adequate to breathing ordinary aii'. The 

 ancestral Isopoda were aquatic animals and their descendants comprisiug the modern family of the 

 Oniscidae have acquired the terrestrial lialnt of life by a gradual process. It would appear that 

 the modificatious 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 di'y 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. Fh'st, 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 

 secm-es also a retardation in the escape of the water of respii'ation — that passing oft" from the 



Bibliütheca zoologica. Heft 25. ** 



