520 Miss Hilda M. Bishop on the 



and efferent edges being about equal in length (fig. 5). In 

 C. europcea the afferent edge is short and the leaflet has a 

 long line of attachment to the mantle, so that only a small 

 part of the leaflet is actually free (tig. 6) . 



The afferent edge of a leaflet is that which is turned 

 towards the branchial roof, and the efferent edge faces 

 ventrally, i. e. lies uppermost when the roof of the branchial 

 cavity is turned back. 



The structure of the leaflets is similar in the two species 

 ^fig. 8). Skeletal tissue stretches along each edge of the 

 leaflet from the efferent edge, where it is considerably thick- 

 ened, but dies out at a short distance from this edge before it 

 reaches the afferent edge. At the point of attachment of 

 the efferent edge to the branchial roof the skeleton of one 

 leaflet is continuous with that of the next. Tlie thickening 

 of the skeleton forms a definite skeletal rod, covering which 

 is a layer of cells some of which are apparently glandular, 

 mucus-secreting, and others are no doubt sensory. Stretching 

 down from the thickened part of the skeleton on each side of 

 the leaflet is a layer of rather low squarish cells with central 

 nuclei, possessing cilia on their free surface. The cilia of 

 one leaflet, coming into contact with those of the next, keep 

 the leaflets apart, and so prevent the ctenidium from col- 

 lapsing. The epithelium of the rest of the leaflet consists of 

 fairly low cells. The internal structure, as in all the other 

 types described, consists of scattered connective-tissue cells 

 with large blood-spaces between them. 



A comparison of Cyprcea with other types as regards the 

 structure of the ctenidium is interesting. 



In Emarginula , the skeleton of the leaflets is similar to 

 that in Cyprcea, but the cells covering the efferent edge do 

 not appear to be glandular and the cilia-bearing cells are 

 much higher, with central nuclei (tig. 7). 



The structure of the leaflets in Haliotis is somewhat similar 

 to that in Emarginula, but cilia are borne also by the cells 

 which cover the efferent edge, so that those cells corresponding 

 to the cilia-bearing layer in Emarginula and Cyprcea are less 

 clearly marked off in this genus (Fleure, 1904). 



In Trochvs, the thickened parts of the skeleton on each side 

 of the leaflet are further apart than in the preceding genera, 

 and Trochus resembles Cyprosa and Emarginula in the fact 

 that cilia are borne only by a layer of cells on each side of 

 the leaflet, and not on the edge as in Haliotis. This layer 

 consists of cells similar to those in Emarginula, but the 

 nuclei are rather basal than central. 



In Pahdina, ihe leaflets are long and narrow, the efferent 



