15 
Where the hypodermic pillars intersect the blood cavity the dorsal and ventral boundary 
walls of the cavity become continuous, forming sheaths which surround the jhllars. The ramifications 
of the tree are also accompanied by folds of the boundary wall of the blood cavity (Fig. 6, W.Bc.). 
In this situation the wall becomes very thin but it may be seen as a clearly defined boundary of 
the communicating blood spaces formed by the divisions of the tree. 
The structure of the tree is well seen in sections taken parallel with the faces of the gill 
(Fig. 7). The opening through which the ramified cavities of the tree communicate with the out- 
side, here appears as a rather narrow passage and the chitinous walls along the passage are sculp- 
tured in the same manner as the specially modified portion of the exterior chitinous wall (Op. tr. ; 
Gr. ch.). The grooved structure of the exterior wall is in fact continued in the infolded portion 
and gradually becomes simplified to the plain wall of the branches of the tree. Tlie passage widens 
into a shallow bowl-shaped cavity from which pass out in radiate arrangment the tulnilar cavities 
of the primary branches of tlie tree (Br. tr.). These divide repeatedly and the final terminations 
approach closely to the walls of the gill (Fig. 6). The mode of branching does not appear to con- 
form to any definite law, excej)ting that the divisions lie at approximately uniform angles with one 
another. Where a division occurs the size of the tubules is reduced only to a moderate extent, 
so that the final branches have a relatively large diameter. The branches terminate in rounded ends. 
The walls of the tubules are thin and show no markings. Lying outside of the chitinous 
layer is the likewise thin hypodermic layer. Outside of this and closely associated with it is the 
boundary wall of the l)lood cavity (Fig. 6; Ch., Hy., W.b.c.). Thus every portion of the wall of 
the tree is composed of three very thin layers. The hypodermic nuclei occur at frequent intervals. 
It is seen from the above that the study of the internal anatomy of the gill establishes 
the conclusions of those authors quoted above who found the cor]>us album to be a branching, tree- 
like cavity communicating with the exterior through an opening. The conception of Duvernoy and 
Lereboullet that the tree is morphologically an inward folding and division of the inner wall of 
the gill is verified. Incidentally to the study of the anatomy it has been shown, as has been hitherto 
generally believed, that the cavity contains air. 
The Physiol ogy of the Gill. In considering the physiology of the parts described 
above it may first be noted that the gill as a whole has no function that is merely relative to the 
inner gills. The name gill-covers usually given by authors to the outer branches of the appendages 
is not applicable to the first and second pairs, since tlie three pairs of inner gills lie within the 
last three pairs of the outer branches. 
As regards the use of names it may be remarked that it seems most suitable to employ 
simply the terms inner and outer gills, using them in their morjihological values. 
The function of the tree and the grooves. It is evident that the gill of Porcellio 
scaber is a structure adapted to bringing the blood into relation with air. Two different and in- 
dependent anatomical structures are employed in order to secure a ready exposure of the two media 
to each other. These are first, the net-work of furrows in the chitine at the exterior of the gill 
and second, the infolded portion of the chitinous wall, forming the internal tree. 
In considering the iihysiological significance of the details of structure of these two parts 
it is helpful to bear in mind the mechanical conditions requisite to securing the general end. These 
are first, an extension of the wall interposed between the two media, in order to afford a large 
