CLASSIFICATION 29 
sponding ones on the two neighbouring filaments, 
just as two brushes may be made to do; these 
are known as the “ciliated junctions” (Plate IV., 
Fig.5,£). Moreover, the two membranes, “lamellz,”’ 
thus arising do not simply depend from the axis, but 
their edges are folded upwards so as to keep them 
within the margin of the shell. The outer lamella 
is folded outwards, and the inner inwards, so that 
they form a W in section (Plate IV., Fig. 5, B). 
A further complication of structure ensues when 
junctions, called “interlamellar junctions,” are formed 
between the dependent part of each filament and its 
reflected portion. In a more advanced stage still 
both the ciliated and interlamellar junctions become 
solid connections, so that the whole structure presents 
a sponge-like appearance, while the reflected ends of 
the filaments, uniting with the walls of the mantle 
and foot, subdivide the pallial chamber (Plate IV., 
Fig. 5, C). One further development there is, in 
which the gills have become converted into a sort 
of party-wall, separating the pallial chamber into a 
dorsal and ventral portion (Plate IV., Fig. 5, D). 
At certain points all over the gill filaments there 
are powerful cilia, which by their action keep a brisk 
current of water flowing over the gills in a constant 
stream from behind forwards. This not only insures 
a fresh supply of oxygen, but conveys the micro- 
organisms on which the animal feeds towards the 
mouth. 
