| OCR Text |
Show 1895.] SENSOEY CANAL SYSTEM OF FISHES. 283 grooves arise, to which special mention will again be made. The canal enters the head just above the angle of the operculum, in which region a dendritic branching is plainly visible and indicates the region of the occipital commissure. Continuing forwards the operculo-mandibular branch passes in a bony canal through the preoperculum and mandible. Slightly in front of this the main canal divides into supra- and sub-orbital branches, the former passing forwards and inwards, and anterior to the nasal capsule, on the lateral border of which it terminates on the side of the nasal barblet. The sub-orbital branch passes around the orbit and forwards, bifurcating between the nares. Course of the Canals and Branches. 1. THE LATEEAL CANAL commences at the base of the fin-rays of the caudal fin, and passes as a fine dermal tube to the region of the head. Opening into the canal at regular intervals are a series of pores averaging from 85 to 95 in number. Previous to entering upon the region of the head a short branch is given off which passes behind the occipital region (PI. XVIII. fig. 1 ) ; a similar branch is given off in front of this, which passes downwards and backwards; both are surrounded by a series of dendritic branches and grooves (PI. XVIII. fig. 5). These grooves present, I think, a stage in the formation of the branches. In the smallest specimens examined they were simple isolated grooves. In later stages they had sunk into the epidermis, and their sides were gradually converging; still later stages showed that the sides ultimately met and formed a minute canal opening by a terminal pore. In the last stage this minute canal becomes connected with the main canal or one of its branches. In addition to these cluster-pores there are, scattered in the immediate neighbourhood of the lateral canal, a series of small papilla-like organs, to which reference will be made later, it being sufficient to here remark that they are not organs in connection with the sensory canal system. 2. T H E M A I N C A N A L enters upon the region of the head above the angle of the operculum. On its inner side it gives off the occipital commissure, and slightly anterior to this a lateral branch- the operculo-mandibular: immediately in front of this the main canal divides into the supra- and sub-orbital branches (PL XVIII. fig. 4). The Supra-orbital Branch is formed by the inner division of the main canal. Passing forwards and inwards in the frontals it gives off a short backwardly-directed branch and numerous dendritic branches terminating in cluster-pores. Slightly anterior to the orbit three small branches directed towards the median line are given off, and a short lateral branch between the nasal openings. A number of fine dendritic branches and grooves similar to those already mentioned (PI. XVIII. fig. 5) are connected with all these. Between the supra-orbital branches of either side of the head are a large number of very small primitive pores, |
