380 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
lamellae, and serving thus to keep the filaments in 
position. 
From this poimt the filaments take up _ their 
position so as to form the folds or plicae, but these are not 
very deep near the base of the lamellae (fig. 22). The 
principal filaments are as described above. Sixteen or 
seventeen ordinary filaments are, on an average, to be 
found between them. Both ascending and descending 
portions of the principal filaments are connected at this 
level by the interlamellar septum, which is very narrow. 
As one passes by serial transverse sections from the 
ventral margin to the ctenidial axis the plications 
increase in depth, and the interlamellar septum is of 
ereater extent until above a point one-third of the height 
of the filaments, the septum dies away in the middle, 
leaving an interlamellar expansion attached to both 
the ascending and descending portions of the principal 
filaments. he general character of the principal fila- 
ments still remains the same. 
As we reach the level at which the ascending filaments 
end (fig. 21), the plication of the reflected lamellae 
decreases, and at the same time the filaments become more 
closely crowded. The principal filaments lose their T 
shape, and become more triangular in section. 
From this point to the upper edge of the lamellae 
the chitinous skeleton of the principal filaments becomes 
more and more reduced, and at the same time the width 
of the filament diminishes, and its diameter from the 
frontal to the interlamellar surfaces increases. The 
ordinary filaments become more compressed and elongated 
as regards the fronto-interlamellar diameter, until the 
final result is that both the principal and ordinary 
filaments look exactly the same, and, owing to the increase 
in diameter, both kinds of filaments become spatulate at 
