Gastropod Foot and Branchial Cavity. 521 



and afferent edges being both long and the line of attachment 

 short, so that they project very freely into the branchial 

 cavity. The structure of the leaflets is very similar to that 

 in Cyprcea, except that here the cells corresponding to the 

 ciliated cells of that genus are much higher, with central 

 nuclei, and cilia are apparently absent or poorly developed. 

 This fact is perhaps correlated with the shape of the leaflet 

 (PI. XIII. fig. 9). The ctenidial leaflets in Littorina differ 

 from all those already described in possessing practically no 

 skeleton. 



In L. Htorea the sides of the leaflet towards the efferent 

 edge are slightly strengthened by a small amount of skeletal 

 tissue. Cilia are borne by a layer of cells on each side 

 corresponding to that in Cyprcea, Paludina, &c, which 

 stretches almost to the efferent adge of the leaflet and is not 

 very definitely marked off from the cells covering that edge. 

 The cilia-bearing cells are rather high and narrow, with 

 basal nuclei (fig. 10). 



In L. obtusata there seems to be no skeleton present, and 

 the cilia-bearing cells are very high and narrow, with basal 

 nuclei (fig. 11). 



In Littorina the afferent edges of the leaflets are continued 

 for some distance along the branchial roof as ridges in its 

 surface. This and the degeneration of the gill-skeleton are 

 modifications probably arising from the high-tide habitat of 

 Littorina. The skeleton of the ctenidial leaflets in Cyprcea 

 resembles most closely that of Emarginula, Halivtis, and 

 Paludina, and the structure is most similar to that of Paludina. 



As would be expected, the structure of the leaflets in 

 L. litorea, the low-tide form, shows less divergence from the 

 primitive type than that of L. obtusata, which lives higher 

 up the tide. 



Prof. G. C. Bourne, discussing the structure of the 

 ctenidium of Septaria, as characteristic of the Neritidee, 

 describes a cilia-bearing region similar to that in the genera 

 described above, and states that there are cilia on the margin 

 of the lamella (cf. the above description for Haliotis). He 

 found, however, no supporting rods or skeletal bars in the 

 lamellae of S<ptaria (Bourne, 1908). 



6. Notes on the Mucus-Glands in the Branchial Cavity 

 of Littorina. 



In Littorina, there is a good deal of variation in the position 

 and extent of the mucus-gland among the different species, 

 and some difference exists between the two sexes in this 



