100 PROFESSOR J. C. EWART AND MR J. C. MITCHELL ON THE 



iug cells to the top of the hillock, and thus they in a way resemble the Miillerian fibres 

 of the retina. 



Besting on the top of the hillock there is often what Solger terms the " cupula- 

 bihlunof." This seems to consist of mucin. In some cases we have seen long threads of 

 mucin extending from the hillock into the cupula or across the canal, the threads having 

 frequently leucocytes entangled between them. 



Each sense organ has a nerve passing to it. The nerves, usually accompanied with 

 one or more capillaries, enter the canal a short distance from their respective sense organs, 

 and run obliquely through the eanal to break up under the hillock into a number of 

 terminal fibres which seem to end in close connection with the hair cells. 



With the exceptions already mentioned, the dorsal canals have the same structure as 

 the lateral canal. 



But, while the sense organs and tubules have a metameric arrangement in the trunk, 

 there is no relation between the sense organs and segments in the head region ; and, as 

 already pointed out, some portions of the cranial canals, though possessing numerous 

 sense organs, have no tubules connecting them with the exterior. 



In all the cranial canals, both dorsal and ventral, there are far more sense organs than 

 segments, e.g., in the supra-orbital canal there are nearly ninety sense organs, and in the 

 infra-orbital there are over ninety. Why the sense organs of the head are so numerous, 

 may be understood should we in course of time discover the function of the lateral sense 

 organs. 



The writers who assert that the sense organs and tubules agree in number have 

 probably only directed their attention to the sensory canals of sharks ; for certainly, as 

 figures 6 and 7 clearly show, there are long stretches of canals with few or no tubules. 

 As to whether in the embryo the tubules are more numerous than in the adult we have 

 no information. If there are more tubules in the embryo than in the adult, it may 

 be inferred that the parts of canals that have lost their tubules are in process of 

 degenerating — of being reduced to vesicles, such as take the place of the ventral sensory 

 canals in the torpedo. 



The Ventral or Subcutaneous Canals. — In these canals, which have usually a lumen 

 five or six times greater than that of the dorsal canals, the wall is of uniform thickness, 

 and composed of a thin layer of fibrous tissue (fig. 9). Like the dorsal, they are lined by 

 two layers of cells, except at the sides of the sense organs. The sense organs only differ 

 from those of the dorsal canals in being slightly larger, and in having a wider zone of 

 columnar cells surrounding them. 



It may be mentioned that in a very young skate we had the opportunity of examining 

 the lining cells, and to a certain extent the sensory cells differed from those of the adult. 

 The two layers of cells which line the canals closely resemble each other ; and, as the 

 sense organ is reached, the layers separate, the deeper one passing under the hillock, while 

 the superficial becomes continuous with the supporting cells. The sensory cells are pear- 

 shaped, and have oval nuclei, while the supporting cells are long and narrow. 



