52() "• li- POLLARD, 



lougitudiually divided iii halves by nicaus of a fretsaw and one half 

 was decalcified in picric acid, stained in Alum carmine and cut into 

 sections of 35 /f thickness. It was then easily possible to construct a 

 diagram of the canals, sense organs and nerves within an outline 

 sketch of the skull and bones drawn on a large scale. The advan- 

 tage of making the first rough outUne very large is that then every 

 section, where there are several hundred, is represented by a readily 

 perceptible and measurable interval. 



The canals in Clarias are as represented in Fig. 1 throughout 

 the greater part of their extent of uniform calibre broadening only 

 where two systems join»). The canals open to the surface by com- 

 paratively few pores, that is to say, never in a dendritic manner. The 

 tubes occupy the centre of the massive dermal bones and pass from 

 one bone to another without alteration in size whether the union of 

 the bones through which they pass is ligamentous as for example of 

 the opercular and squamosal or suturai as of the squamosal and post- 

 frontal. Clarias shows interesting transitions from the more primi- 

 tive ligamentous union of dermal bones to a much indented suturai 

 connection. When a canal is about to communicate with the exterior 

 it gives off towards the surface a branch which varies very greatly in 

 length. Usually the branch runs for a space in a superficial groove 

 of the bone (as for example pore 5 of the supraorbital system) roofed 

 only by membrane. It then leaves the bone running in the dermis 

 where it is enclosed by a tough cellular tissue and finally opens by 

 a pinhole aperture. 



Infraorbital canal. This commences with pore No. 1 be- 

 hind the tubular opening of the nose and above the anterior part of 

 the autopalatine. It runs backward a short distance and opens by 

 another pore No. 2 which lies just in front of the small bone corre- 

 sponding to the antorbital of Amia. Between these two pores lies the 

 first sense organ which is only partially enclosed by rudimentary der- 

 mal bone. The third pore opens between the antorbital and lachry- 

 mal bones. The second sense organ lies within the antorbital and on 

 the upper internal portion of the mucous tube. Sense organs do not 

 necessarily lie at the base of the canals. The fourth pore is situated 

 between the lachrymal and suborbital bones beneath the centre of the 



1) In the terminal portions however of some systems e. g. the supra- 

 orbital, there is a tendency towards reduction to a rudimentary state. 



