— 15 — 



Where the hypodermic pillars intersect tlie blood cavity the dorsal and ventral boundary 

 walls of the cavity become continuous, forming sheaths which Surround the jiillars. 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 seeu 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. eh.). The grooved structure of the exterior wall is in fact continued in the infolded portion 

 and gradually becomes simplified to the piain wall of the branches of the tree. The passage widens 

 into a shallow bowl-shaped cavity from which pass out in radiate arrangment the tubulär cavities 

 of the primary branclies of the tree (Br. tr.). These divide repeatedly and the final terniinations 

 approach closely to the walls of the gill (Fig. 6). The mode of branching does not appear to con- 

 form to any definite law, excepting 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 Idood cavity (Fig. 6; Ch., Hy., W.b.c). Tlius 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 corpus 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 Physiology of the (Jill. In considering the physiology of the parts described 

 above it may fii-st 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 ])y authors to the outer branches of the appendages 

 is not applicable to the first and second pairs, since the 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 morphological values. 



The function of the tree and the grooves. It is evident that the gill of Porcellio 

 seaber is a structure adapted to briuging the blood into relation with air. Two different and in- 

 dependent anatomical structiu-es 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 physiological 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 



