25 



The relative areas covered by the different structures 

 are indicated in Text-fig. 3. 



The connective tissue supporting skeleton extends 

 from the axis along the efferent border to the angle. At 

 first it extends from the free edge of each leaflet to the 

 mantle, but it gradually becomes reduced. The area 

 of ciliated cells increases quickly and then extends to the 

 angle of efferent and afferent edges, occupying in sections 

 from about a third to one-sixth of the length of the leaflet 

 (measured from efferent margin to mantle side). The 

 glandular cell area is nearest to the mantle. It increases 

 steadily in thickness as the afferent edge of the leaflet is 

 approached and is widest in that region. 



Fig. 3. ct. e.'" Area of Glandular Cells, ct. e." Area of Ciliated Cells. 

 These differ from the reference letters on Plate IV. 



One further detail remains to be described. Peculiar 

 characteristic bridges run across the blood cavity in the 

 leaflet. Each of these appears to be formed of muscle 

 fibrils which diverge at their extremities (fig. 34, 

 Ct. mus.). The cell in which these muscular fibrillae 

 have been formed remains, and is usually quite obvious 

 with its residual cytoplasm and nucleus in the centre of 

 the bridge. 



Thus it is possible by contraction of these numerous 

 muscle strands to approximate the two surfaces of the 

 leaflets, and hence to force out the contained blood. 



