— 14 — 



general cavity of the gill and contaius blood (B.c.). The blood cavity is boimded liy a thin wall 

 which lies iminediately witbin tlie bypodermis (AV.b.c). 



In tbe special portion of the gill there are no pillars Init the blood-cavity is penetrated 

 by 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 openhig (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 uumerous small communicating Spaces, containing blood (B.sp.). But at the base of the tree 

 and surrounded externally by that portion of the chitinous wall which is scidptured into fui-rows 

 that contain air is a large space or Chamber (B.ch.). The quantity of blood contained in tliis 

 Chamber gives the appearance of an accumulation of blood 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 difi'erent parts of the 

 gill. It is best developed in the general part where the nuclei are large and surrounded with cell- 

 protoplasm. In the sj)ecial part, aloug the two walls of chitinc, the layer becomes thinner and the 

 nuclei are flattened. Along the infolded portion of the chitinous wall, forming the tree, the cell- 

 protoplasm is greatly reduced and the layer becomes very thin, except where the nuclei occur (Hy. 

 Nu.). The nuclei are everywhere marked by clearly delined boundaries and are highly granulär. 

 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 which are 

 elongated in the direction of the axis of the pillar. 



The blood cavity occupies the whole space within the hypodermic layer and is bounded l)y 

 a thin wall lying contiguous to the hypoderm and conforming to its irregularities (W.b.c). In this 

 wall at long intervals lie elongated nuclei. I infer, from the relations of this wall that it is of 

 mesodermic origin. ' 



'■ Since the present treatise was written my attention lias been called by Professor Chun to the werk of Leich- 

 mann, Beiträge zur Naturgeschichte der Isopoden, published in the Bibliotheca zoologica, Heft 10. 

 Cassel 1891. It appears from this werk that the bi'ood-sacks of the Isopoda are in their general features homologous 

 with the gills. Äs shown by this author the brood-sacks arise as folds of the hypoderm and at an early stage of deve- 

 lopment the hypoderm cells beeome gronped in such a way as to leave a net-work of Spaces between the opposite walls 

 of the sack. A cross-section of the brood-lamella of Astilus aquaticus (a fignre of which is given) taken at this stage 

 of development shows an identity in structnral plan with that of the gills of the Oniscidae, in the cases where the latter 

 have nndergone no special modifications in adaptation to air-breathing, as in the last three pairs of outer gills of PorcfW/o 

 scaher. According to Leichmann, the net-work of Spaces in the brood-lamellae which, as in the gills, contain blood, are 

 lacnnose. He fignres these Spaces as bounded by a clearly deflned 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 delinite wall surrounding the blood-spaces and probably of mesodermic origin. I was inclined to this 

 view 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 band LEICHMANN flnds in the brood-lamellae 

 no such layer bnt 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 would 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. 



