4(; 



enclosing a space between them and the darkly staining 

 portion. This pale staining variety of chitin has a more 

 fibrous structure than the homogeneous central portion 

 (fig. 23, Ch. L.). 



The nerve branch passes down one side between the 

 dark staining chitin, where it approaches the wall of the 

 filament, and the epithelial cells, but between the nerve 

 and the latter is a very delicate strand of chitin or 

 connective tissue. 



Several strands of tissue cross the filament, and the 

 interlamellar margin is extended to form a membrane, 

 which towards the lower free margins of the principal 

 filaments extends completely across from the descending 

 to the reflected portions. 



The interlamellar junctions (fig. 23, Br. j. I.) which 

 unite the two lamellae only extend about one-third of the 

 height of the filaments, though the principal filaments 

 develop interlamellar extensions along the greater part of 

 their length. 



This interlamellar extension, however, has a dift'erent 

 character in two places. It is well developed on the 

 descending portion of the principal filaments above the 

 level of the free upper edge of the reflected lamellae, that 

 is to say, in a position where there are no opposite prin- 

 cipal filaments for it to be connected with (fig. 20, Br. r.). 

 This expansion, which is found on each principal filament 

 at its end nearest to the ctenidial axis, will be described 

 below. It diminishes until it practically disappears at 

 about the height of the upper ends of the reflected 

 filaments, and then on the same side another expansion 

 develops, which, more ventrally still, extends across to 

 meet and fuse with its fellow on the opposite principal 

 filament and form the interlamellar septum. 



The first described expansion (fig. 2U, Br. r.) may be 



