8 OYSTERS AND DISEASE. 



Each gill filament has the branchial epithelium (ectoderm) on its outer surface, 

 and the blood channel, lined by a thin layer of connective tissue, in its interior (Fig. 4). 

 The skeletal bars lie between the epithelium and the connective tissue; the blood space 

 contains here and there leucocytes, rounded or stellate, for the most part colourless, blood 

 cells. There are then the following four tissues : — 



1. The branchial epithelium. 



2. The skeletal bars. 



3. The connective tissue. 



4. The blood cells ; and also certain eosinophilous cells. 



The Epithelium. — This varies in appearance in different parts, and requires further 

 description. It has been figured and described most satisfactorily in the past by 

 Janssens* and by De Bruynef, and we also show it in Figs. 4, 5, and 10, on PI. I. 

 A tract of the epithelium on each side of each filament bears especially long cilia, and 

 these tracts are separated by a narrow non-ciliated zone from the front or free edge 

 of the filament, which is covered by a uniform band of short cilia (Fig. 5). The 

 depression between adjacent filaments, where they join, is never ciliated. The nuclei of 

 all these epithelial cells are large and ellipsoidal (Fig. 10). The cells on the prominent 

 free edge of the filament are of tall, columnar form ; at the base and on the interfilamentar 

 junctions they are cubical, squamous epithelium. Amongst these epithelial cells we find 

 the large rounded granular cells, which were described by Lankester as secretion cells, 

 by J. Chatin as " macroblasts," and by others as " Beckerzellen." They are clear and 

 refracting in the white oyster, but distinctly green -coloured in the green oyster. They 

 give a marked eosinophilous reaction (Fig. 5), and will be referred to again below in 

 that connection. 



The Skeletal Bars lie two in each gill filament, inside the epithelium and near 

 the centre, in a section {see Fig. 5, etc.). They diverge outwards towards the outer 

 edge of the filament, and either join or come into close proximity in the middle line 

 of the free edge. They occasionally come in contact at their inner ends also {see Fig. 5). 

 In the trough between two of the series of projections formed of filaments, the larger 

 modified gill filament (the " segment primaire " of Janssens) has a pair of especially 

 thickened skeletal bars of wedge-like shape, in section, and set at a wide angle to one 

 another (Figs. 2 and 3). It is only these larger gill filaments, and usually only the 

 alternate ones of these, that are continued into inter-lamellar junctions (Fig. 3). 



The Connective Tissue forms a thin layer, in which the skeletal bars lie, inside 

 the epithelium, and bounding the blood-channels in the branchial filaments and their 

 junctions. It is a membrane nearly homogeneous or slightly fibrillated, with occasional 

 fusiform or stellate connective-tissue cells, the nuclei of which are seen stained in the 



* Les Branchies des Acephales. La Cellule, t. IX. fasc. i, p. 19 (1893). See also Kellogg, U.S. Fish Commission 

 Bulletin, vol. X. (1892). 



+ Arch. de Biologic, t. XIV. (1895). 



