48 



lamellae, and serving thus to keep the filaments in 

 position. 



From this point 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 

 princij)al 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 

 greater 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. The 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 



