264 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



sequently the direction of the canals can only be approximately 

 determined by the study of the surface. 



Within recent years the study of the course of the mucous canals 

 has received an impetus from the discovery of their relation to the 

 morphology of the skull, and accordingly it will be found detailed 

 in Prof. McMurrich's paper on the osteology of this species. 



The canals in the head vary considerably in their dimensions ; 

 their diameter is on the whole greater, sometimes twice as great as 

 that of the lateral canal, and their walls are different in so far as 

 the protective canal is formed of true osseous substance throughout. 

 Except in respect to the greater size of the nerve-hillocks, the lining 

 epithelium appears to be very similar. A transverse section through 

 a nerve-hillock from a young specimen is represented in Fig. 7. The 

 upper half of the tube is occupied by the ordinary epithelium, which 

 becomes thicker as it approaches the neuro-epithelium, projecting 

 inwards so as to lessen the cavity at this place. Two kinds of cells 

 are to be distinguished in the neuro-epithelium : sensory cells, short 

 and oblong, occupying the inner half of the height of the epithelium, 

 and indifferent cells (Stuetzzellen) occupying the whole height with a 

 basal nucleus. The latter are more frequent at the point of passage 

 into the ordinary epithelium. Fig. 8 l'epresents a section of a macula 

 acustica from a fish of the same age, drawn under similar conditions ; 

 the resemblance of the two kinds of neuro-epithelia is particularly 

 striking. In Fig. 7 the whole height of the neuro-epithelium is 37 fi, 

 of the sensory cells 15*5 fi ; the nuclei of these are 6-5 //., of the 

 indifferent cells 4-5 fi. The latter stain very densely in carmine, con- 

 trasting with those of the sensory cells in this respect. Here and 

 there between the indifferent cells are structures which are possibly 

 nerve fibres in section. 



To return to the course of the canals in the head. It will be 

 observed from Fig. 6 that the lateral line rises as it passes forwards 

 towards the posterior upper angle of the gill-cover. Before reaching 

 that a short tube is given off which opens in the skin over the 

 ascending process of the supraclavicle. Directly over the posterior 

 upper angle of the gill-cover is another pore (Figs. 4 and 6) and in 

 front of that another. At the plane of the latter the canals of the 

 two sides communicate by the ' occipital commissure,' which again 

 has two apertures near the middle line. The canal proceeds forwards 

 from this plane, and again opens by a short tube over the articula- 



