1895.] SENSORY CANAL SYSTEM OF FISHES. 293 



anterior portion it makes a slight dorsal curve and terminates on 

 the region of the head by opening a wide saccular dilatation. The 

 pores distributed along the surface of the canal are numerous, 

 averaging in the posterior half of the body 7 to every 2 cm. 



2. The Main Canal of the Head. — As previously mentioned, at 

 the point where the lateral canal terminates there is a wide 

 saccular dilatation, out of which the main canal opens. As soon as 

 this dilatation has been cut open the entrance to the main canal 

 appears as a definite circular opening. Its border is surrounded 

 by a ring of cartilage, and the rest of the canal by a series of 

 cartilages similar to that figured on PI. XXI. figs. 18, 19. In very 

 large and old specimens this cartilage becomes partially ossified. 

 The canal passes inwards and downwards and through the frontal 

 bone, returning to the surface again slightly posterior to the orbit, 

 and divides into the supra- and sub-orbital branches. There 

 are no other branches passing off from the main canal, and no 

 pores opening to it from the surface of the head. The diameter of 

 the canal is very irregular ; in the posterior portion it is fairly wide, 

 but narrows considerably previous to passing into the frontals. 

 In the anterior portion, prior to its division into the two orbital 

 branches, it widens again. 



The Su23ra-07-bital Branch passes inw^ards and forw'ards after 

 leaving the main canal, and as an almost straight branch passes to 

 the tip of the snout, where it opens by a large pore posterior and 

 dorsal to the anterior nares (PI. XXI. figs. 16, 17). 



The Sub-orbital Branch passes behind the orbit and some distance 

 below it ; before passing forwards beneath the orbit a large saccular 

 dilatation passes backwards and opens by a large pore (PI. XXI. 

 figs. 16, 17). Passing forwards to a region above and slightly 

 in front of the angle of the mouth, another but shorter dilatation 

 passes off, and, like the former, opens by a pore ; still further 

 anterior is a third, into which the finger can be readily inserted. 

 There is no pore opening from this. The branch now enters a 

 cartilage and passes upwards and then downwards and forwards, 

 and terminates by a large pore beneath the opening of the supra- 

 orbital branch. 



Tlie Operculo-mancUbular Branch commences immediately at the 

 termination of the lateral canal. At its very commencement and 

 just below the wall of the lateral canal is a pore which leads into a 

 large saccular dilatation which passes backwards and slightly 

 ventrally, terminating blindly. Passing downwards, enclosed in a 

 series of cartilages, it again widens at its base into another much 

 larger dilatation, which opens to the surface by a pore ; slightly in 

 front of this is another dilatation, somewhat smaller (PI. XXI. 

 fig. 17). The branch continues forwards, and, passing inwards, 

 opens into the mandible, through which it passes, opening to the 

 siu-face by four pores and meeting with its fellow of the opposite 

 side. The connection is formed by a short canal in the cartilage. 



The mandible is peculiarly modified for the reception of the 

 mandibular portion of the operculo-mandibular branch, a large 

 portion of the ramus being broken up into a bony network, 



