14 
general cavity of the gill and contains blood (B.c.). The blood cavity is bounded by a thin wall 
which lies immediately within tlie hypodermis (W.h.c.). 
In the special portion of the gill there are no pillars hut the blood-cavity is penetrated 
l)y a system of branching canals. This system is in fact a single much divided cavity, tree-like in 
form, and communicating with the outside of the gill through an opening (Op.). In other words, it 
is morphologically an infold of the wall of the gill. 
It is evident that this tree-like structure is a section of the corpus album. 
The tree, as a whole, divides that portion of the blood-cavity of the gill in which it lies 
into numerous small communicating spaces, containing blood (B.sp.). But at the Imse of the tree 
and surrounded externally by that portion of the chitinous wall which is sculptured into furrows 
that contain air is a large space or chamber (B.ch.). The quantity of blood contained in this 
chamber gives the appearance of an accumulation of l)lood corpuscles, referred to above. 
Minute Internal Anatomy. The cellular structure of the hypoderm is indicated only 
by the nuclei, no cell-walls being visible. The layer varies in thickness in different parts of the 
gill. It is best developed in the general part where the nuclei are large and surrounded with cell- 
protoplasm. In the special part, along the two walls of chitine, the layer becomes thinner and the 
nuclei are flattened. Along the infolded portion of the chitinous Avail, forming the tree, the cell- 
protoplasm is greatly reduced and the layer becomes very thin, except Avliere the nuclei occur (Hy. 
Nu.). The nuclei are everywhere marked hy clearly defined boundaries and are highly granular. 
The pillars consist of hypodermic tissue and at their ends are structurally continuous with the 
respective dorsal and ventral hypodermic layers. They contain one or more nuclei Avhich are 
elongated in the direction of the axis of the pillar. 
The blood cavity occupies the whole space within the hypodermic layer and is hounded by 
a thin wall lying contiguous to the hypoderm and conforming to its irregularities (W.b.c.). In this 
Avail at long intervals lie elongated nuclei. I infer, from the relations of this Avail that it is of 
mesodermic origin. ' 
^ Since the present treatise Avas written my attention has been called by Professor Chun to the work of Leich- 
MANN, Beit rage znr Naturgeschichte der Iso p ode n, published in the Bibliotheca zoologica, Heft 10. 
Cassel 1891. It appears from this Avork that the brood-sacks of the Isopoda are in their general features homologous 
with the gills. As shoAvn by this author the brood-sacks arise as folds of the hypoderm and at an early stage of deve- 
lopment the hypoderm cells become grouped in such a way as to leave a net-work of spaces betAveen the opposite walls 
of the sack. A cross-section of the brood-lamella of Asellus aquaticus (a figure of which is given) taken at this stage 
of development shows an identity in structural plan with that of the gills of the Oniscidae, in the cases where the latter 
have undergone no special modifications in adaptation to air-breathing, as in the last three pairs of outer gills of Porcellio 
scaber. According to Leichmann, the net-work of spaces in the brood-lamellae which, as in the gills, contain blood, are 
lacunose. He figures these spaces as bounded by a clearly defined line, but he regards this line as merely the boundary 
of the hypodermic walls and pillars. In my description of the gills, as above given, I have considered this boundary 
line as forming a definite wall surrounding the blood-spaces and probably of mesodermic origin. I was inclined to this 
vieAv from the fact that nuclei occur, though very sparsely, along these lines where they immediately adjoin the hypo- 
dermic wall. 
According to my conception a layer of mesodermic origin which bounds the body-cavity extends into the gills 
and forms a boundary wall of the blood-spaces of the gills. On the other hand Leichmann finds in the brood-lamellae 
no such layer but considers that the spaces are lacunae and that the blood lies immediately in contact with the hypo- 
derm. The clearly defined line surrounding the spaces is the inner marginal line of the hypodermic tissue. Leichmann 
speaks of the “innere Chitinlamelle“. 
In view of this discrepancy I wmild leave as undetermined in the present work the nature of the wall in question. 
I hope at a future time, through the study of the embryology of the gill, to determine the point finally. 
