078 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
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 
(higet2a "Ch. Ee) 
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. 7. d.) 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 different 
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. 20, Br. r.) may be 
