EPITHELIAL CELLS— HYDATINA 133 



quite briefly of the gills of Ganvrmms in the description of a 

 figure referring to it. 



An opportunity of making some studies upon the Eotifer 

 Hydatina senta enabled me to observe the very beautifully 

 striated and fibrillar nature of the protoplasm of the large 

 cells which carry the cilia of the two ciliated circles of the 

 wheel organ. The cells of the posterior circlet are especi- 

 ally well suited for this observation. Both in animals 

 subjected to pressure, as well as in such as had been 

 narcotised with hydroxylamine, it is very easy to con- 

 vince oneself as to this structure in the epithelial cells. As 

 far as I could see the entire protoplasm is not differentiated 

 into fibrils, but only a middle layer, from which the cilia 

 arise externally. The higher and deeper layers, on the other 

 hand, have the structure of an irregular reticulate meshwork, 

 and at the same time contain numerous peculiar elongate 

 or somewhat kidney-shaped bodies, which are very closely 

 packed in the framework, and remind one greatly of Bac- 

 terioids (Plate VII. Fig. 5). The fibrils of the middle 

 layer run down deep into the large cell until they approach 

 the conspicuous nucleus, without, however, reaching the 

 base of the cell. They are very distinctly knotted, and 

 closer observation demonstrates with certainty that the 

 nodes of neighbouring fibrils are connected by delicate 

 transverse threads. The structure is thus seen to consist 

 of a meshwork so disposed as to give a striated appearance, 

 and not to be really fibrillar. 



The surface of the cell is limited (at least where the 

 cilia arise) by a radially striated border, possessing to some 

 extent the characteristics of an alveolar layer. The striae of 

 this border are direct continuations of the fibrillse, and pas's, 

 on the other hand, immediately into the cilia externally. 

 In photographs which I have taken of such cells, I further 

 notice at their inner circumference a fairly distinct alveolar 

 layer, as well as a radiate layer of meshes round the 

 nucleus. 



The anterior circlet of cilia consists, as is well known, of 

 a number of tufts of cilia. To each of these tufts corre- 



