OCR Text |
Show 276 MB. W. E. COLLINGE ON THE [Apr. 2, the frontal region; it continues through the nasal and terminates in the premaxilla. The sub-orbital branch passes through the anterior border of the sphenotic and into the post-orbital, in which it gives off a short branch and passes into the sub-orbital, giving off another branch and also opening by a pore. It continues forwards in the pre-orbital, in which in the suture with the nasal there is another pore; it then passes through the nasal bone and terminates in a bifurcation in the maxilla. Course of the Canals and Branches. 1. THE LATEEAL CANAL is an exceedingly fine dermal canal, scarcely visible to the naked eye in the posterior portion of the body. In the anterior region its diameter increases slightly; in no portion, however, was it found in transverse diameter to exceed *6 of a millimetre. It commences at a small pore 5 millim. in front of the base of the rays of the caudal fin. This terminal portion of the canal is directed somewhat dorsally, and therefore shares in the upward flexure of the terminal part of the vertebral column, as in the tail of Polyodon. It continues forwards along the side of the body, rising upon the post-temporal region. Distributed along its whole length are a series of pores, which are largest and most plentiful in the anterior region. 2. T H E M A I N C A N A L of the head commences in the squamosal. At its commencement it gives off a lateral and backwardly directed branch (PI. XVIII. fig. 1) which ends blindly, a feature common to very many of the Physostomi. In front of this branch the operculo-mandibular branch arises. The main canal then continues forwards, passing through the dorsal border of the sphenotic. Its course is not perfectly straight, as in many fishes, but has a slight lateral inclination. In the most anterior portion of the sphenotic the canal divides into supra- and sub-orbital branches. The Supra-orbital Branch.-Leaving the main canal on the anterior border of the sphenotic this branch passes forwards and inwards in the frontal. At its commencement it gives off on its inner side a backwardly directed branch, which passes into the squamosal and terminates at pore 15 (fig. 1). In front of this branch a much smaller one (pore 16) passes off and opens on the dorsal surface of the head. Slightly in front of this, and about the centre of the frontal bone, a branch (fig. \,f.com.) is given off which meets with its fellow of the opposite side, thus forming a frontal commissure and connecting the supra-orbital branches of either side. A similar commissure is present in Chcetostomus. From this commissure the canal continues through the frontal and lateral ethmoid, in the latter giving off a lateral and backwardly directed branch terminating at pore 17 (fig. 1), and passes into the nasal, here also branching laterally. The main branch passes forwards into the premaxillae, on the lateral border of which it terminates at pore 19. |