23 



bounding layer of epithelium enclosing a flattened cavity, 

 which is a blood space. Underlying this epithelial layer 

 (figs, 31, 32, 33, ct. eJ, ct. e." ', ct. e.'") there is an internal 

 connective tissue layer with muscles, lining the blood 

 space just mentioned. Between these two layers a 

 supporting tissue is developed which is much thickened 

 near the ctenidial axis and runs along the efferent margin 

 of each leaflet (figs. 31, 32, 33, Sup. m.). 



This supporting tissue, which in its thickened parts 

 is apparently hyaline, has been regarded as cartilaginous 

 or chitinous; this will be referred to below. 



The relation of the various structures enumerated 

 can be made out best in a section transverse to the leaflets 

 and perpendicular to the mantle. Such a section, running 

 through the efferent margin, is figured on PI. IV (figs. 

 31,32,33). 



The external epithelium, which bounds the lamellae, 

 differs considerably in the various regions. Taking the 

 section through a point near the efferent margin, there is 

 first the free edge to be considered. The epithelium here 

 is composed of somewhat deep and regular columnal 

 cells bearing cilia (fig. 33, ct. eJ). This epithelium 

 diminishes in thickness and becomes more irregular than 

 indicated in the figure as one leaves the free margin. 

 The cilia are also lost and the cells appear glandular 

 (fig. 33.) Following on this region the cells again become 

 more deep, more regular and with deeply staining cyto- 

 plasm. They all bear well developed cilia, so that the 

 whole area occupied (fig. 32, Ct. e.'") by them is very 

 conspicuous by reason of the marked contrast with the 

 glandular cells lacking cilia on either side of it. Passing 

 this area towards the mantle, the cells become once more 

 glandular and without cilia. The epithelium here is 

 often thrown into folds through contraction, probably on 



