22 SYDNEY EVANS JOHNSON 



caudal peduncle. Even in large adults the lateral canals ter- 

 minate in open grooves on the sides of the caudal fin. 



Figure 4 is a diagram to show the relations of sensory canal, 

 surface tubules, lateral nerve and its ramuh. The ramuli of 

 the nerve as well as the tubules have been much foreshortened in 

 order to bring them within the limits of the diagram. 



The sensory epithelium of the lateral canal of Mustelus forms 

 a colunm that is substantially continuous throughout the length 

 of the canal (figs. 5, 6, 7, 8). It is not divided into segments 

 corresponding to the metameres of the trunk. This sensory 

 column occupies the dorsal wall of the canal. It is two or three 

 cell layers in thickness and it is in this sensory colunm that we 

 find the development of sensory cells that are called the sense 

 organs of the lateral line. Figure 5 is a semi-diagrammatic 

 sketch of the sensory colunm between two tubules. There are 

 16 clusters of cells (outlined with the camera) between the 

 two tubules. The ramuli of the nerves correspond to the tubule 

 and the nerve fibers overlap. 



The thickness of the lateral sensory column is not constant 

 throughout its length, but becomes gradually thinner poste- 

 riorly (figs. 9, 10, 11). It is also thinner as a rule midway be- 

 tween two ramuli of the lateral nerve. This may indicate a 

 tendency for the sensory column to assume a segmental char- 

 acter, but in no instance in Mustelus has the sensory column 

 been observed to be completely divided into separate 'buds' or 

 segments, as appears to be the condition in Amia and several 

 other forms. 



On each side of the lateral sensory column are large blood 

 spaces and the adjacent columnar epithelium of the canal wall 

 is considerably thickened (fig. 8). Little tubules lead from the 

 canals to the surface of the integument at approximately regular 

 intervals. Their walls are composed of two layers of epithelial 

 cells which are continuous with those of the inferior wall of the 

 canal (fig. 6). The tubules coincide in number with the ramuli 

 of the lateral nerve and they do not increase in number after 

 the canals have closed. A hundred and thirty-six tubules were 

 counted in the lateral canal of a specimen 55 mm. long, while 



