Rectal Trucheal Gitls. 215 



011 tlie structiire of the gill of Libcllula depressa, hcre are to be nientioned 

 some diverse resiilts of iiiy present study oii L. 4-maciilata. The whole 

 surface of the gill is covered by a thick cuticiila, iinder which the Single 

 layer of hypodermal cells is found, and the structure and arrangement 

 of the cells vary according to various parts of the gill. Those in the 

 laniella are niuch flattened, each cell containing a small, oblong nucleus 

 and inany pigment granules; the space between two layers of opposite 

 side is generaliy so narrow that it is scarecely able to contain any thing 

 more than the finest trachii, which often penetrate into the cytoplasma 

 of the cells. At the basal cushion, two opposite layers are not only 

 widely separated, bnt also show a different structure of cells, of 

 which they are composed ; the layer on the farther lateral froni the 

 median line of the double row is composed of cells similar to those 

 which compose the lainella, while the layer near the median line is chiefly 

 made up by a set of larger cells containing larger nuclei (fig. 3). These 

 peculiar larger cells are grouped in a roundish patch, whcnce the name 

 „Bourrelets epitheliaux" was given by Sadones (fig. 2, EC). 



The Cells of the Epithelial Cushion. 



The cells forming the epithelial cushion of a gill are not other than 

 the cells of the hypodermal epithelium arranged in a unicellular layer, 

 but peculiarly modified. They vary in nuinber according to the individual 

 gills. All of these cells take their arrangement of several concentric rings 

 around the imaginal centre of the cushion, forming a roundish or 

 frequently eiliptical disc. The size of a Single cell as well as of its 

 nucleus also varies according to the position they occur. At the central 

 pait of the cushion the cells are the tallest, and the nuclei are generaliy 

 roundish in shape, while at the periphery the cells are niuch flattened 

 and then uclei are rather like a slender rod (fig. 7.) Those cells and nuclei 

 which lie intervcning between these two zones show an intermediate 

 form, the iatter being in this zones largest in size. The elongated 

 nucleus lies near the periphery and its long axis is parallel to the cushion 

 surface (fig. 7), a fact just contrast to the case of L. depressa investi- 

 gated by Sadones. Therefore on any transverse section of a cushion, 

 there may be found the nuclei which are roundish or elongated on 

 section, as they are at the right angles cut trough their long axesparallel 

 to the surface, and on a tangential section of a cushion, all of thcm 

 show an elongated shape except those in the centre. In regard to contents 

 of a cell one is able to recognize without difficulty three distinct parts 

 of the protoplasm; the part nearest to the periphery is finely striated; 

 the next zone which surrounds the nucleus, is more or less granulated, 

 while the third or the innermost part is occupied by a fibrous structure of 

 protoplasm. The first of these three is doubtless homologous to the intima of 

 the epithelium of mid-intestine, but in this cell is the thin and transparent 



