122 ME. F. J. COLE ON THE STRUCTURE AND MORPHOLOGY OF 



Strictly speaking, and arguing from its condition in those fishes in which its innervation 

 has heen carefully ascertained, it should form a dorsal connection hetween the anterior 

 extremities of the lateral canals. It may, however, interpolate itself, as it does in 

 Cliimcera and other recent fishes, and perhaps also in Pterichthys, hetween the two 

 orhital canals, so that these do not anastomose posteriorly, hut hoth arise from the lateral 

 canal. But, however this may he, no canal can he considered a supra-temporal canal 

 which is not innervated by the lateralis nerve, and which therefore is not a portion of 

 the latei-al canal. 



I hence propose to describe the sensory canals of Oadus morrJiim in the following 

 order : — (1) Supra-orbital canal ; (2) Infra-orbital canal ; (3) Hyomandibular canal ; 

 (4) Lateralis canal. All these canals will be described from before backwards. 



D. Sensory Canals of Gabus morrbua. (PI. 21, fig. 1.) 



(1) Supra-orbital Canal. 



The supra-orbital canal commences blindly at the anterior extremity of the nasal bone, 

 and therefore of course at the extremity of the snout. This blind extremity appears 

 very late in the ontogeny of the system, and is not seen in sections of young Gadids — the 

 canal here being directly continuous Avith the first dermal tubule (I propose to designate 

 as " dermal tubules " those small tubes by which at intervals the main sensory canals 

 communicate with the surface *). The supra-orbital canal on the nasal bone pursues a 

 slightly sigmoid course obliquely inwards, upwards, and backwards, and measures whilst 

 on this bone about 40 mm. long. The nasal is deeply grooved to receive the base of the 

 canal, and its inner edge is jmrtly turned outwards to form a rough tube — imperfect, 

 however, dorsally and externally. Anteriorly the bony nasal furrow is imperfect ventro- 

 lateially, in order that the fii'st dermal tubule may enter the supra-orbital canal (see 

 PI. 21). The second dermal tubule enters the supra-orbital canal 25 mm. from the 

 posterior extremity of the nasal, and lies in a backwardly-directed oblique furrow situated 

 on the lateral wing of the nasal. 



The canal now leaves the nasal and for a distance of about 5 mm. has no bony floor, 

 but only a ligamentous one. An internal wall and roof are, however, partly supplied by 

 a forward projection of the frontal. The third dermal tubule enters the canal at this 

 region, but somewliat nearer the frontal than the nasal. 



The remainder of the supra-orbital canal on the frontal is more conveniently described 

 from behind forwards. In front of its anastomosis with the infra-orbital it courses 

 inwards and forwards for a distance of about 20 mm. It then enters by a lateral 

 foramen a canal bored in the substance of the frontal. Tiie median portion of this canal 

 forms a perfect tube, but posteriori}^ the roof is imperfect owing to the jiresence of a 



* The dermal tuLules and sense organs of the lateral canals are enumerated in order from before backwards 

 in full knowledge of Allis's important proof of the relation between the sense organs and the tulnilcs. Any 

 scientific enumeration of these structures in the ease of the Cod is, however, impossible until the development of 

 both has been investigated, and any attempt therefore to do so here would be purely arbitrary and misleading. 



