SENSORY CANALS OF THE COMMON SKATE. 91 



As it runs along the side of the rostrum it gives off the twenty branches for the second 

 (rostral) part of the canal ; while, as it passes through the cartilage in front of the orbit, 

 it gives off a large branch (v.b., fig. 7) that curves outwards and downwards to break 

 up into the forty-five twigs for the ventral portion of the supra-orbital canal — viz., sixteen 

 for the inner part, twelve for the outer part, and seventeen for the looped part. This 

 branch of the superficial ophthalmic comes into intimate relation with various branches 

 of the buccal nerve. 



II. The Infra-Orbital Canal. — This canal (I.O., figs. 6 and 7), which, is continuous 

 with the cranial portion of the lateral canal, and in communication with the supra-orbital 

 canal, runs obliquely outwards between the eye and the spiracle, and then bending 

 forwards runs for some distance nearly parallel to the supra-orbital canal. It next 

 curves forwards and outwards to the margin of the snout, which it pierces to reach the 

 ventral surface. It then runs backwards, communicating on the way with the supra- 

 orbital, and on reaching the hy ©mandibular (HM., fig. 7) turns inwards, to dip into 

 the naso-buccal groove. Emerging from the groove it soon unites with the corresponding 

 canal of the opposite side, and forms a very short median portion (I.O. 7 , fig. 7) from 

 which two canals curve sharply outwards, and then running forwards in close contact 

 with the suborbital canals again unite near the tip of the snout (I.O. 8 , fig. 7). 



The infra-orbital canal includes the orbital, suborbital, orbito-nasal, nasal, half of the 

 median, and the prenasal canals of Garman. The first (orbital) part of the infra-orbital 

 canal seems to be a direct continuation forwards of the lateral canal. It has the same 

 diameter and structure as the lateral canal. For the first 4 or 5 mm. of its course, it lies 

 in a groove in the cartilage of the cranium, and afterwards is embedded in the fibrous 

 tissue between the eye and the spiracle. As it bends forwards outside the eye it gives 

 off a large tubule (t. 1 , fig. 6),* which runs backwards external to the spiracle, and opens by 

 a terminal pore to the exterior. The portion of the canal which runs forwards in front of 

 the eye (I.O. x -L0 2 ) lies immediately underneath the skin,, and slightly increases in size 

 before passing to the ventral surface. The ventral portion may be said to consist of 

 three parts : — (1) A straight part continuous with the hyomandibular canal, and in com- 

 munication with the supra-orbital (I.0. 4 -L0. 5 ) ; (2) a looped part (L0. 5 -I.Q. 7 ), which 

 runs inwards to the middle line posterior to the nasal capsule ; and (3) an inner part 

 (I.0. 7 -L0. 8 , fig. 6 ; I.O., fig. 6a), which runs forwards almost in contact with the inner 

 ventral part of the supra-orbital (S.O. 1 , fig. 6a). These three ventral segments lie em- 

 bedded in the gelatinous tissue of the snout from 2 to 4 mm. from the surface, and possess, 

 as a whole, a considerably larger lumen than the dorsal portion of the infra-orbital. 



As is the case with the supra-orbital, some parts are provided with, while other parts are 

 without, tubules. The tubules in front of the eye (t.-t. ls , fig. 6), twelve in number, spring 

 from the dorsal part of the canal. At first under a centimetre in length, they gradually 

 increase up to two centimetres. Four of them (3-6) have a peculiar arrangement ; they 

 connect the infra-orbital canal with the dorsal part of the hyomandibular (H.M. 5 , fig. 6), ; 



* This tubule Garman considered a branch of the main canal — a view not supported by it., structure. 



