nv 
Fig. 17 A drawing, made under high magnification, of a section through one 
of^^the sense organs of a canal of type 1. Only a small portion of the epithelium 
near the median line of the canal is shown. 
14 and 14a lie further below the surface, so that, in the regions 
between the enlargements, a cross section has the appearance of a 
simple alveolar gland, with a duct of considerable length (fig. 14a). 
As the stratified epithelium of the surface extends down into 
this deep groove it becomes reduced in thickness until, in the region 
where it passes between the upper ends of the cartilage rings, it 
consists of only a few layers of cells. At this point the super- 
ficial cells of the epidermis are suddenly converted into a layer 
of rather short columnar cells: while the lower cells of this epi- 
dermis become continuous with (though they could scarcely be 
said to be converted into) the thin layer of loose, fibrous tissue 
that lies between the epithelium of the canal and its cartilaginous 
and fibrous rings. This transition from the superficial to the lining 
epithelium is quite sudden and is fairly distinct, even in the tis- 
sue at hand. 
In the wide regions of the canals the superficial epithelium above 
the cartilaginous rings is modified to form the glands (fig. 14, g), 
that have been mentioned, but the transition into the columnar 
