SENSORY CANALS OF THE COMMON SKATE. 93 



and run forwards all but in contact with the terminal portion of the ophthalmicus 

 superficialis — the division of the facial with which the buccal was originally united. 

 Two of the branches which proceed outwards from the inner division of the buccal 

 supply the part of the canal lying between its junction with the hyomandibular canal 

 and the inner margin of the naso-buccal groove. The part outside the groove receives 

 seven twigs, the part which dips into the groove, six. Another branch breaks up into 

 nine filaments for the part of the canal between the groove and the short commissure. 

 This short expanded median portion receives as many as ten relatively large branches — 

 five from the right and five from the left buccal nerve. The inner (prenasal) portion of 

 the infra-orbital canal receives altogether twenty-six nerves from the inner division of 

 the buccal as it runs along the rostrum ; eleven of these, which reach the curved part of 

 the canal behind the tubules, are relatively large, while the fifteen which pass to the 

 anterior two-thirds are more slender and further apart. It thus appears that the buccal 

 nerve innervates nearly one hundred sense organs — ninety-eight terminal twigs having 

 been traced in the specimen examined. 



The infra-orbital canal, though containing nearly one hundred sense organs only, 

 opens to the exterior by forty-nine tubules. The two canals communicate with each 

 other in front of the mouth, and also, as in Amia, at the tip of the snout, and by means 

 of four of its tubules with the hyomandibular canal ; while, by its proximal end, it is 

 continuous with the lateral and supra-orbital canals. In communicating with the lateral, 

 and in having a commissure at the tip of the snout,* the infra-orbitals of the skate differ 

 from those of Lsemargus ; they also differ in having a limited number of tubules — far 

 fewer tubules than sense organs. 



III. The Hyomandibular Canal. — The hyoid portion of this canal, as in Lsemargus, 

 is absent, but the distal part of the mandibular persists. The horizontal portion, on the 

 other hand, has been greatly extended, forming a long loop under the pectoral fin ; and, 

 with the help of an offshoot from the lateral, a long wide loop on the upper surface. 

 Beginning where the infra-orbital bends inwards, the hyomandibular (HM., fig. 7) runs 

 backwards, external to the branchial clefts, as represented in figure 7, to form the long 

 ventral loop (HM. 1 , fig. 7). The outer limb, when opposite the mouth (HM. 2 ), curves 

 inwards, and then runs forwards immediately outside the infra-orbital canal, to pierce 

 the tissues inside the propterygium and reach the dorsal surface. Having gained the 

 dorsal surface, it at once dilates, to form in some cases a club-shaped expansion (HM. 5 , 

 fig. 6) — the end of which is connected, as already explained, by tubules with the infra- 

 orbital canal. From this expansion the canal, now comparatively slender, curves out- 

 wards and backwards for some distance along the margin of the pectoral fin, and then 

 bends inwards to join a branch (scapular ; sc, fig. 6) from the lateral canal. This greatly 

 extended hyomandibular corresponds to the angular, jugular, subpleural, and pleural 

 canals of Garman. 



The inner limb of the ventral loop gradually diminishes in size as far as the middle of 



* The union of the prenasals at the tip of the snout is said by Garman not to take place in B. Icevis. 



