SENSORY CANALS OF THE COMMON SKATE. 99 



and roof agree in consisting of a thin layer of fibrous tissue with at the most a few car- 

 tilage cells. The roof is translucent ; hence, when a canal is exposed, the position of the 

 lumen is at once evident, presenting quite a different appearance from the dense lateral 

 walls. 



The thin, fibrous roof and floor, and the thick, fibro-cartilaginous lateral walls, and the 

 sense organ, are shown in figure 8. The sense organ (s.o.), it will be observed, stretches 

 almost right across the inner wall. Usually the tubules run obliquely through the 

 wall, and the fibro- cartilage extends along each tubule to within a short distance of 

 the external aperture. Beyond the cartilage the tubule consists of epidermal cells, 

 with pigment cells interspersed ; these pigment cells, by forming a dark ring, often 

 indicate the position of the terminal pore. 



The sense organs throughout the greater part of the lateral canal do not, as might be 

 expected, lie in the floor ; but, as shown in figure 8, in contact with the inner lateral 

 wall. In front of the shoulder-girdle they are either on the side of the canal or on the 

 roof. 



The canals (with the exception of the parts occupied by the sense organs) and the 

 tubules are lined with two layers of epithelial cells. The deeper layer consists of rounded 

 and somewhat irregular cells which rest on a basement membrane, and are often separated 

 by intercellular spaces containing leucocytes. 



The superficial cells are columnar in form ; in most cases they are short and broad, 

 having the free outer end non-granular, and the deep end occupied by the nucleus. 

 These columnar cells, though resembling mucin-forming cells, were never seen assuming 

 the goblet form, or giving evidence of being actively concerned in the production of 

 mucus. They contrasted strongly with the goblet cells which exist in great numbers in 

 the epithelium of the skin (fig. 8) ; and, undoubtedly, produce the abundant coating of 

 mucus always present in the skate. 



In longitudinal and transverse sections through the areas occupied by the sense 

 organs, it is observed that, as the sense organ or hillock is approached, the deeper layer 

 of cells disappears, while the superficial layer assumes the form of long narrow columns, 

 each with a nucleus near the middle of its length. These columnar cells form_ a well- 

 marked zone around the sense organ proper — a zone sometimes fifteen cells wide. The 

 sense organ which lies within this zone consists of sense cells, supporting cells, and of 

 highly refractile processes, which project into the hillock from the basement membrane. 



The sense cells, which are of a cylindrical form, lie within and between the support- 

 ing cells. Each has a large nucleus near its deep, inner end, and a hair-like process 

 projecting from its outer end. These hair cells are less numerous than the supporting 

 cells, which lie between and around them. The supporting cells, especially towards the 

 centre of the hillock, are long and narrow, and thus differ from the short and compara- 

 tively broad cells which line the canal and the hair cells already mentioned. The pro- 

 cesses which project into the hillock from the basement membrane Solger describes as 

 " zwischen-pfeiler" (9). They seem to extend from between the inner ends of the support- 



