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, E). Moreover, the two membranes, " lamellae," 

 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. 



