48 



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 stifi" 

 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 discs, for they have no organic connection, but 

 are held somewhat firmly in position by the great develop- 

 ment of interlocking cilia. 



Each 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 28j, 

 the chitin is thinnest on the margins facing the pallia! 

 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, Fit. Sep.), is generally spoken of as being 

 chitinous. In most of my sections where it is seen 

 distinctly, it 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 lining 

 the tube of the filaments on the inner side of the chitinous 



