260 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



When viewed in transverse section (PL III., fig. 1) the 

 inner half of the canal is seen to be lined by very long 

 cells possessing large nuclei and with an inner hyaline 

 extremity consisting of fused cilia, at the base of which 

 are minute deeply-stained granules. On the outer side 

 of the canal the cells are even more highly specialised, 

 and are five in number (as seen in transverse section), 

 viz., a median one, two smaller ones on either side, and 

 two very large ones again on either side of these. More 

 than one nucleus is present in all of them. The cells of 

 these five rows are separate at their inner ends both from 

 one another and from the cells of the internal half of the 

 canal. Like the latter they possess an inner hyaline 

 portion, consisting of fused cilia projecting into the lumen 

 of the canal. The basal portions of all these cells are 

 without a well-marked limiting' membrane, and come into 

 close contact with the fibrous core of the posterior 

 extremity of the dorsal ganglion. Numbers of small 

 sensory ganglion cells of the first type ( page 16 ) are 

 massed round the canal (PI. I., fig. 6, and PI. II., fig. 4), 

 and the projection of the fibrous core from the dorsal 

 ganglion. 



The function of this elaborate organ is still pro- 

 blematical. By some writers it has been supposed to 

 contribute to the respiration of the brain lobes, though 

 the specialised character of its epithelium and the number 

 of ganglion cells in it would seem to lend more counte- 

 nance to the view that its function is rather concerned 

 with the elaboration of some special sensory impulses. 



(2) The frontal organ consists of three small projecting 

 patches of high columnar glandless epithelium bearing 

 cilia (PI. IV., fig. 1, /V.). The median patch is situated 

 just above the proboscis pore with a lateral patch on 

 either side of it. These patches are retractile, and after 



