The Lateral Line System in Siluroids. 527 



eye. The third seose organ lies in the lachrymal. The fourth sense 

 organ lies in the suborbital where the raucous canal begins to curve 

 up round the back of the orbit and just behind this sense organ arises 

 the branch which leads to the fifth pore. This branch possesses a 

 special importance and will be referred to in the comparison with 

 other forms. It passes ofi backwards and slightly downwards lying 

 in a groove of the suborbital and roofed in only by membrane. It 

 opens between the suborbital and large postorbital. 



The suborbital canal continues its course obliquely upwards through 

 the large postorbital, the fifth sense organ lying not far from the centre 

 of this bone. A little behind the centre of the bone another branch 

 is given off also lying in a groove of the bone and roofed in by mem- 

 brane. The sixth pore is situated between the postorbital, postfrontal 

 and supratemporal (Huxley). 



The suborbital canal joins the supraorbital in the postfrontal bone 

 and is then continued as the main line of the body. 



Supraorbital canal. The first pore of this system is situated 

 just medianly to the external tubular opening of the nose. It leads 

 to the canal lying in the nasal bone. The second pore lies at the 

 side of this bone comparatively close to the first. Between the two 

 is the first sense organ enclosed in the substance of the bone in the 

 posterior part of which lies also the second sense organ. The canal 

 then passes into the prefrontal (dermal) bone and shortly gives ofi" a 

 branch which leads to the third pore. The branch runs in a groove 

 for some distance backwards almost parallel to the mucous canal. The 

 latter continues its course straight posteriorly lying close to the edge 

 of the peculiar depression of the frontals. The third sense organ 

 lies on it at some distance posteriorly to the level of the eyes. The 

 next branch given oô from the canal shows this peculiarity that it 

 does not open to the exterior but runs for some distance towards the 

 middle line. It is distinctly rudimentary and gives very strongly the 

 impression that it formerly united with its fellow of the opposite side. 

 It will be necessary to refer to this point later. 



Close behind this transverse branch lies the fourth sense organ and 

 in i)roximity to it the corresponding fifth branch and pore. This branch 

 runs in a groove of the bone above the course of the mucous canal and it 

 is noteworthy that it does not open near any suture. In Clarias it is 

 by no means a rule that pores should open at sutures. The fifth 

 sense organ is placed only slightly posterior to the above branch, the 

 third, fourth and fifth being fairly close together and not far from 



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