288 MB, W. E. COLLINGE O^ THE [-^pr- -. 



observers ; thus M'Donnell (9. p. 175) describes the lateral canal 

 as consisting o£ a series of isolated follicles each opening to the 

 surface by a pore. Even so careful an observer as Leydig (7. p. 33) 

 speaks of it as an interrupted canal, possibly being misled by the 

 series of interrupted scales. Eamsay Wright (12. p. 265) also 

 refers to it in like terms. 



Xot^Adth standing these observations, there is a distinct and very 

 large lateral canal in Esox. It commences about 12 millim. from 

 the base of the caudal fin and passes as a wide tube as far as the 

 supraclavicle. Its greatest transverse diameter is 5 millim. and its 

 smallest 1 millim. It opens to the surface by a series of pores, 

 each lying in a little pit. The canal lies beneath the scales in the 

 dermis. The sensory organs lie slightly anterior to the pore. 



In transverse section the canal exhibited the usual structure. 



2. The Main Canal of the Head. — This canal enters the skull 

 in the pterotic, through which it passes as a wide canal. M'Donnell 

 (9. p. 175) states that the cephalic portion and the lateral line 

 proper are not connected with each other. Probably he overlooked 

 the portion traversing the supraclaAacle and the Y-shaped canal- 

 bone. The main canal terminates at the anterior end of the 

 pterotic, giving rise to the supra- and sub-orbital branches. 



The Sujpra-orbital Branch lies in the frontal bone. It is a simple 

 wide canal opening to the surface by four large pore-like openings. 

 From the frontal it passes into a canal-bone lying on the lateral 

 border of the frontal, and terminates some little distance in front 

 of the nasal capsule. 



T7ie Sub-orbital Branch first passes through a small oval-shaped 

 ossicle lying upon the sphenotic, and continues its course through 

 a series of canal-bones surrounding the orbit, in front of which it 

 makes an upward turn and terminates by opening to the surface 

 external to the nasal capsule. 



The Opercular Branch. — Unlike the condition found in most 

 fishes, the opercular branch is quite distinct f lOm either the main 

 canal of the head or the mandibular branch. Commencing at the 

 head of the preoperculum as a large pore, it passes through this 

 bone to its distal end, where it terminates by a similar pore. In 

 its course through the bone it gives off three small branches, each 

 of which opens to the surface by a pore. 



The Mandibular Branch passes through the greater portion of the 

 mandible as a closed canal opening to the surface by three pores, 

 as well as one at its commencement and termination. It terminates 

 about 13 millim. from the symphysis. 



In Esox there are no true commissures connecting the canals of 

 one side of the head with the other, but a series of open grooves 

 upon certain regions of the head probably take their place. 

 Behind the occipital region these are most prououuced. They 

 commence on either side of the head from the pore on the inner 

 arm of the Y-shaped canal-bone previously mentioned. Some 

 portions of these grooves anastomose with each other in the 

 median line, thus forming a connection between the canals of each 



