SEA-FISHERIES LABORATORY. Bie 
situated on the sides of the filaments near the inter- 
lamellar margin, forming little cushions, and these are 
found at regular intervals down each filament, those of 
adjacent filaments being, as a rule at the same level. The 
walls of the filaments are thus brought much closer 
together, and the narrow space is occupied by the stiff 
interlocking cilia which these cells bear. 
The upper ends of the ascending portions of the 
filaments (fig. 21, Br. a.’) are almost entirely large, 
ciliated dises, for they have no organic connection, but 
are held somewhat firmly in position by the great develop- 
ment of interlocking cilia. 
Hach of the filaments, ordinary and principal, is 
strengthened by a skeletal support formed of chitin. Thus 
the skeleton of the gill can be prepared by acting on the 
soft tissue with warm caustic potash. 
In the ordinary filaments this skeleton is a simple 
tube lining the inner surface of the epithelium (fig 23), 
the chitin is thinnest on the margins facing the pallial 
and interlamellar cavities respectively, and thickest on 
the sides. At one place on each side, nearer the inter- 
lamellar margin than the frontal margin, the chitin tube 
is much thicker, and a septum arises here and crosses the 
filament dividing the tube into two parts. 
This septum, known as the Intrafilamentar Septum 
(fig. 23, Fil. sep.), is generally spoken of as being 
chitinous. In most of my sections where it is seen 
distinctly, 1t appears to stain quite differently from the 
chitinous skeleton, appearing almost as if it were cellular, 
and nuclei have been seen in it which were not adhering 
blood corpuscles. : 
There has been much discussion regarding the 
presence of a continuous layer of endothelial cells hning 
the tube of the filaments on the inner side of the chitinous 
