O74 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
junctions, and so the various filaments can be separated 
easily in the ving animal. Organic interfilamentar 
junctions occur in P. tenwicostatus (1), which makes with 
Avicula argentea and Margaritifera vulgaris (9) a third 
exceptional member of the Hleutherorhabda. 
The histology of the gills has been worked out from 
serial sections cut chiefly parallel to the ctenidial axis, 
that is transversely through the filaments, but in addition 
sections have been cut parallel to the filaments in two 
directions. 
Perennyi, or Mann’s Fluid, can be used for fixing the 
gills, and then Methyl-blue-eosin differentiates the 
chitinous framework well, staining it blue, the cytoplasm 
purple and the cilia bright red. The principal filaments 
should be dissected out, stained, cleared and mounted in 
Canada Balsam iv order to make out the various paris. 
The filaments are tubes bounded by a very delicate 
epithelial wall, the cells of which differ in shape and size 
at various points. 
The ordinary filaments (fig. 25, /’7l. 0.) have deeper 
cells round the margin further away from the inter- 
lamellar space, and these cells are deepest in three places 
forming longitudinal ridges, the cells of which bear cilia. 
Thus in transverse sections of the filaments there will be 
seen three seis of cilia on the filament, one down the 
frontal surface—the frontal cilia (fig. 23, C. fr.)—and 
one on each side of the filament—the lateral cilia (fig. 23, 
C..), a narrow space separates the frontal from the lateral 
ciha. This disposition of cilia is characteristic of most 
of the Lamellibranchiata. These cilia, unlike those of 
the ciliated discs, are, in life, in constant action producing 
the currents of water on which the animal depends for its 
food supply and oxygen. 
The ciliated discs are patches of elongated cells 
