GILLS OF LAMELLIBRAJVCH MOLLUSCA. 63 



those of Posner on this head. Figs. 20, 21, 22, 23, in 

 Plate VI, give my results most accurately, and where the 

 other figures of Anodon (in Plate V) differ from these, such 

 differences are to be regarded as inaccuracies or diagrammatic 

 licences. We can distinguish on the gill-filament a frontal 

 epithelium (/■.e.),latero-frontal {l.f.e".) and lateral epithelium 

 (le, le), the last-named continuing into the water-passages and 

 clothing the interlamellar surfaces. All the cells of the 

 epithelium are ciliated, those of the latero-frontal rows (a 

 single row on each side the frontal epithelium) have the 

 longest cilia, far out reaching those of the other cells ; the 

 frontal epithelium and the more forward cells of the lateral 

 epithelium come next with finer and much shorter cilia; and 

 lastly, the in-lying lateral epithelium has but very short cilia. 



The cells of the line of latero-frontal epithelium deserve 

 especial notice. Posner speaks of them as *' einfach durch- 

 borten," but entirely omits to notice the fact that they are 

 very much larger than the neighbouring cells, as shown in 

 fig. 23. When seen from the surface of the lamella each 

 latero-frontal cell appears like a goblet-cell with a single 

 coarse flagellum issuing from it in place of a mucous pro- 

 duct (see fig. 21). A^ hen turned over or seen in the trans- 

 verse section of the filament the cell is seen to be broad and 

 a little flattened, the single flagellum proves to be an adhering 

 group of long cilia, which apparently issue from the mouth 

 of the goblet- like cell by a series of pore canals, that is to 

 say, there is a cork or ' Propf to the cell as to intestinal epi- 

 thelium, but it is traversed by these cilia. The nucleus is 

 large and clear and deeply placed with but a small amount 

 of stainable protoplasm around it. Staining with picro- 

 carmine colours the nuclei of all the epithelial cells intensely 

 and also a small amount of surrounding protoplasm, but the 

 whole cell is not deeply coloured. Hence the appearance 

 presented by the frontal epithelium in fig. 20, and in the 

 similar view of Dreissena filaments in Plate Vll. 



The form and arrangement of the epithelial cells will be 

 best understood by reference to PI. VI and its explanation. 



I have found absolute alcohol to the fresh Anodon, and 

 mounting the sections in glycerine, to give better results as 

 to the epithelium than does osmic acid. 



Lacunar Tissue. — In PI. VI, fig. 18, is gisen a view very 

 highly magnified of what Mr. Lankester considers a typical 

 piece of lacunar tissue. It is from a sub-filamentar outgrowth. 

 The nuclei of the original cells are seen surrounded with a 

 small quantity of granular protoplasm which gradually 

 gives place to a slightly metamorphosed substance, forming 



