21 



distance between the anterior and posterior ends of that sys- 

 tem, and to thus enable the fish to appreciate and determine 

 the direction of its migration according to the temperature 

 differences, is an open question. While such a suggestion 

 may seem far-fetched and improbable, there is enough of the 

 barest possibility of the lateral line system having such a 

 function to warrant serious consideration. 



The other point in relation to the lateral line system, which 

 seems to me to be of sufficient interest to be worth noting, is 

 the following : Every one, upon carefully examining the sides 

 of the head and opercles of the shad for the first time, will 

 have the attention arrested by the large amount of clear tis- 

 sues in front of and behind the eyes and over the gill-covers, 

 forming a quite considerable layer over the latter, which is 

 traversed in its deeper parts by five canals, which open to the 

 surface. Similar canals traverse tissues covering the space 

 between the eyes over the front of the head. In these canals, 

 many of which are exceedingly narrow and repeatedly 

 branched before reaching the surface, the cephatic portion of 

 the lateral line system is lodged. The terminations of the 

 nerves are at the bases of little clusters of cells, adherent to 

 the walls of the canals, surmounted by five hair-like proto- 

 plasmic processes. These fine processes of the cells are prob- 

 ably the terminal elements of an exceedingly delicate special 

 sensory apparatus. The fine processes project into the fluid 

 in the canals, and when the fluid is set in motion by even the 

 slightest vibration, sensory impressions are conveyed to the 

 brain of the fish. In the present state of our knowledge it is 

 probably useless to speculate as to the uses of this delicate 

 mechanism, which is many times more complex than the sys- 

 tem of labyrinths found in the human ear. Some idea of the 

 complexity of the system of canals may be obtained by care- 

 fully inspecting the manifold branchings of this system on 

 the opercles, where they may be readily made out with a 

 pocket magnifier, embedded in the considerable layer of clear 

 substance already mentioned. 



An examination of this clear substance with the aid of sec- 



