SENSORY CANALS OF L^MARGUS. 71 



the hyomandibular, either as a ventral offshoot or as an independent canal, developed 

 from a branchial sense organ immediately behind the one in connection with which the 

 infra-orbital canal originated. 



The supra-orbital offshoot may have been developed as the eye increased in size ; 

 while the hyomandibular offshoot or canal is in all probability correlated to the mandible, 

 and, when present, the operculum. It is, however, extremely probable that from the 

 first the main canal of the head forked on reaching the orbit, and thus formed supra- and 

 infra-orbital canals. 



The accessory or proximal portion of the infra-orbital canal in Lsemargus may be 

 known as the otic part (ot., fig. 1). Though continuous with the anterior part of the 

 lateral canal, it is not continuous with the infra-orbital. It may be considered as repre- 

 senting that part of the canal system in Amia containing the sense organs numbered 15 

 and 16 (fig. 3), which are supplied by the otic branch of the facial nerve. 



The otic canal in the specimen of Lsemargus under consideration is 2*5 cm. in length. 

 It begins 1 cm. behind and slightly further from the middle line than the common 

 terminal pore of the supra- and infra-orbital canals. It then bends outwards and back- 

 wards, and on the way gradually sinks to a distance of 4 mm. from the surface, and 

 becomes continuous with the cranial portion of the lateral canal.* The canal is penetrated 

 by four branches of the otic nerve, and it opens to the surface by four tubules, the anterior 

 tubule having a length of 3 '5 mm., the others increasing in length from before back- 

 wards — the fourth measuring 7 mm. — the increase being partly due to the deeper position 

 of the canal, and partly to the posterior tubules running obliquely outwards through the 

 skin. 



The part described as the otic canal I expected at first to find supplied by a dorsal 

 branch of the glossopharyngeal nerve — a nerve which, even in Amia, takes part in 

 innervating the lateral sense organs. Hitherto, however, I have failed to trace any 

 branches from the glossopharyngeal to either sensory or ampullary canals in Lsemargus. 

 It is, however, possible that the most anterior portion of the lateral canal — the part 

 immediately continuous with the otic — is supplied by fibres which spring from the glosso- 

 pharyngeal nerve before it leaves the cranial cavity. 



Innervation of the Infra-orbital Canal. — The otic portion is supplied by fibres which 

 leave the buccal nerve immediately after it separates from the superficial ophthalmic. 

 As the buccal passes outwards and forwards to run under the eyeball it gives off branches 

 for the first portion of the infra-orbital canal proper. It then divides into two main 

 branches, the inner of which supplies the sense organs of that part of the canal which 

 runs inwards and forwards under the rostrum, while the outer division supplies the sense 

 organs of the intermediate part of the canal. 



3. The Hyomandibular Canal. — This canal (HM., fig. 1) which, as already 

 explained, is only partially represented in Lsemargus, consists of a canal about 10 cm. in 

 length, which runs backwards and outwards from the infra-orbital canal. This part of 



* This is supposing the glossopharyngeal nerve sends no branches to the sense organs. 



