GILLS OF LAMELLIBRANCH MOLLUSCA. 53 



epithelial cells, forming what may be called epithelial pronii- 

 iiences (Plate IV, fig. 1). These cells carry long cilia which 

 interlock, as might the hairs of two brushes, with the cilia 

 of an epithelial prominence on the neighbouring filament. 

 The band-lilie character of the filament and the circular 

 outline of the epithelial prominence of a ciliated junction is 

 well seen in fig. 3. It is not unusual for a filament to twist 

 and turn over in this way when separated by teazing from 

 the support of neighbouring filaments to which it is held by 

 the interlocked cilia. The series of band-like filaments when 

 viewed from the surface of the lamella may be compared to 

 the vertically placed planks which form the sub-structure of 

 a flooring. In fig. 1 the small portion of three filaments 

 which is drawn is represented as seen in somewhat deep 

 focus — deep enough, in fact, to give an optical section in the 

 plane of the ciliated junctions. A superficial focus of the 

 narrow surface of the filament would resemble that of 

 Mytilus drawn in fig. 5 (left-hand one of the three filaments). 

 The broad diameter of the gill-filaments in Area, that is, the 

 breadth of the surface on Avhich are the ciliary junctions 

 measured from the lateral (outer) to the medial (inner) border, 

 averages ttti^^^ of an inch. The narrow diameter measured 

 at the surface of the lamella is the gxoth of an inch. 



To ascertain the true structure of the gill-filament in Area, 

 as in all cases, it is necessary to cut a transverse section of 

 it. Such sections may be obtained by embedding gill-lamellse 

 hardened in absolute alcohol, in paraffin. The sections may 

 be advantageously examined in glycerine without the use of 

 staining fluids, or the lamella may have been stained in 

 picrocarmine before embedding, and mounted after cutting 

 in balsam. On the whole I find that glycerine gives a better 

 definition of cilia and fibrous structures in the various 

 Lamellibranch gills than does balsam. 



In fig. 2 a transverse section of a gill-filament of Area is 

 represented. It is seen to possess a single layer of epithelial 

 cells, all of which are ciliated ; those on the outer or lateral 

 border (upper of the figure) being larger and columnar in 

 form, and carrying longer cilia than those on the inner or 

 median and anterior and posterior borders. A group of cilia 

 of great length is placed on each side of the anterior border 

 of the filament ; the cilia may be called the latero-frontal 

 cilia (PI. IV, Ifc, fig. 2), and the single row of epithelial 

 cells which bear them are the cells of the latero-frontal epi- 

 thelium. The histological details of this epithelium I have 

 worked out more fully in the case of Anodon and Dreissena. 



The section given in fig. 2 does not pass through a ciliated 



