1895.] SENSOEX CA>"AL SYSTEM OF FISHES. 283 



grooves arise, to which special mention vn\\ 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. SHghtly in fi-ont 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 Lateral Cakal 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. The Mais' Canal 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 (PI. XVIII. 



fig- 4)- 



The Sujpra-orhital Branch is formed by the inner division o£ 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 gf very small primitive pores. 



