46 



delicate nerve (fig. 23, Fil.7i.) runuing longitudinally 

 down tlie filament. It is connected with the branchial 

 nerves to be described later. 



The principal filaments differ in shape according to 

 the level at which the transverse sections are cut. 



If sectioned at about the level of the terminations of 

 the ascending filaments, they have a T shape, owing to 

 the frontal surface being much extended in an antero- 

 posterior direction, the vertical limb of the T representing 

 the interlamellar portion (fig. 21, Fil. p.). 



The principal filaments have no connections other 

 than by ciliated discs with the adjacent ordinary filaments. 

 The ordinary cilia are to be found chiefly on two 

 longitudinal ridges of columnar cells near the anterior 

 and posterior edges of the frontal surface, but very short 

 cilia are borne by all the cells on this surface. 



The chitinous skeleton of the principal filaments is 

 much more extensive and complicated than that of the 

 ordinary filament. The most conspicuous and strongest 

 part consists of two bars, somewhat triangular in section, 

 which run longitudinally down the middle of the frontal 

 surface just underneath the epithelium. The two edges 

 of these bars nearest the frontal surface are fused, the 

 chitin has a homogeneous structure and stains very deeply 

 (fig. 23, Ch. D.). 



From the outer sides of these two thickenings a strip 

 of the same dark staining chitin extends in an inter- 

 lamellar direction until it eventually reaches the walls of 

 the filament, which it lines for some distance. 



Extending from the sides of the two longitudinal 

 bars, near the frontal surface, are two thick lateral 

 expansions of a paler staining kind of chitin which 

 extend, lying against the epithelium, along the whole 

 extent of the frontal siirface, and also line tlie sides, 



