SEA-FISHERTES LABORATORY. Sel 
delicate nerve (fig. 23, F7l.n.) running longitudinally 
down the 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, Fel. p.). 
The principal filaments have no connections other 
than by cilated discs with the adjacent ordinary filaments. 
The ordimary 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 1s 
much more extensive and complicated than that of the 
ordinary filament. The most conspicuous and strongest 
part consists of two bars, somewhat triangular im 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 surface, and also line the sides, 
