OCR Text |
Show 288 MB. W. E. COLLINGE ON THE [Apr. 2, observers; thus M'Donnell (9. p. 175) describes the lateral canal as consisting of 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 tbe series of interrupted scales. Ramsay Wright (12. p. 265) also refers to it in like terms. Notwithstanding 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. T H E M A I N C A N A L 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 supraclavicle 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 Supra-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. The 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 i-oni 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 pronounced. 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 |