19 



AVlieu vicwpfl ill iraiisvoi'.so secHoii (PI. III., ficr. 1) the 

 iiinor half of tlie canal i.s seen to l)e lined liv veiv long- 

 cells })osses.sing lavg-e nuclei and with an inner hyaline 

 extiemitv consistinpf of fused cilia, at the 1)a!>e of which 

 are niiiuit(> dei^ply-stained o'ranules. On the outer side 

 of the canal the cells are even iuor(^ higdih- specialised, 

 and are ti\e in nuniher (as ^eell in transverse sec'tion), 

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

 two very larg-e ones again on either side of these. More 

 than one nucliMis 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 

 poition, consisting of fused cilia projecting- into the lumen 

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

 without a well-niaiked limiting memlnane, and come into 

 close contact with the tibrous core of the posterior 

 extremity of the dorsal ganglion. A'unibers of small 

 sensory ganglion cells of the tirst type ( page 1() ) are 

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

 and the projection of the tibrous core from the dorsal 

 ganglion. 



The fuiK-tion of this elaborate organ is still j)i'o- 

 blematical. liy some writers it has been supposed to 

 contribute to the respiration of the brain lobes, though 

 the specialised character of its epithelium and the numlier 

 of ganglion cells in it Avould 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 (PL lY., fig. 1, fr.). The median patch is situated 

 just above the proboscis pore with a lateral patch on 

 either side of it. These patcdies are retractile, and aftei' 



