262 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



(b) Nerve-Hillocks. 



(1). Sense Organs of the" Lateral Line and of thf, 'Mucous' 

 Canals of the Head. 



The system of cutaneous canals which lodge from place to place 

 the sensory nerve-hillocks was at one time described as the system of 

 ' mucous ' canals, owing to the belief that the skin owes its slimy 

 surface to the secretions of these. It is now very well known that 

 the sliminess is due to the mucus-cells described above, and that 

 any mucus which is found in the interior of the canal system has the 

 same sort of relation to the nerve-hillocks as the endolymph in the 

 auditory labyrinth to the macula? acusticce. The fact that the canal 

 system has a very free communication with the outside, renders it 

 probable that the surrounding medium must penetrate it in such a 

 way as to dilute any mucus present. 



The canal system in Amiurus possesses the arrangement which is 

 •commonest among Teleosts, that is to say the canal of the lateral 

 line is entirely imbedded in the cutis, and opens only from place to 

 place by the pores, while it communicates anteriorly with the more 

 complicated canal system of the head. In other Teleosts the scales 

 of the lateral line are modified in various ways both by the presence 

 of the canal and its pores, but as these are entirely absent in 

 Amiurus, the pores are simpler in their structure. It is very much 

 easier to study the apertures of the canal in the fresh condition than 

 in a preserved specimen, owing to the absence of pigment in the 

 immediate neighbourhood of the pores, and to the fact that their 

 edges are somewhat swollen. 



All of the lateral pores are similar in character, with the exception 

 of the two terminal pores, which are near the caudal fin, and which 

 open obliquely into a small detached portion of the canal. This is, 

 no doubt, a relic of the interrupted lateral canal seen in other 

 Physostomous forms e.g. Esox. Forty pores are present on each 

 side : as the number of pores corresponds to the number of nerve- 

 hillocks (although opening into the canal at some little distance from 

 these), and the spinal nerves ai*e also present in the same number, 

 it is obvious that the sense organs of the lateral line are disposed 

 in a metameric fashion here as in other Teleosts. 



The lateral canal corresponds exactly in position to the cleft 

 between the dorsal and ventral divisions of the lateral musculature. 



