464 H. J. FLEURK AND MURIEL M. GETTINGS. 



channels^ ruus across the roof of the cavity iu the region of 

 these attachments. 



In Trochus this connection of the afferent side of the gill 

 axis with the roof of the cayity has extended much farther 

 forwards. Tin's has raised the lower series of gill-leaflets so 

 that they hang in the cavity instead of almost^ if not quite, 

 resting on its floor. In this new position they are more 

 efficiently bathed by the incoming water, and are less likely 

 to impede its course, and they are also more easily kept from 

 " packing together." The dorsal series of leaflets is, however, 

 necessarily enclosed in a pocket through this development, 

 and, in the species considered, tliis series is reduced as 

 compared with the more freely hanging ventral leaflets. 



In dettiiled structure the cteuidial leaflets of Trochus 

 resemble in the main those of Haliotis (fig. 6). They are 

 epithelial folds with a foundation of elongated cells below 

 the epithelium of each surface, and bridges of cells across 

 the cavity, which is a blood space. Between the epithelium 

 and the cells which more or less line the blood space, Ave 

 find, as in Haliotis, a development of chitinous substance, 

 the ends of the chitin plates towards the " efferent " border of 

 ctenidial leaflet being thickened. Away from this thick part, 

 the chitinous layer thins out, and is soon no longer observa- 

 ble; but, where the efferent border of the leaflet meets the 

 efferent side of the gill axis, it can be seen that, as in 

 Haliotis, the chitinous plate of one side of a leaflet is 

 continuous with that of the opposite face of the next leaflet. 

 A nerve runs along each border of the leaflet beneath the 

 epithelium. The epithelium (fig. G) along the efferent border 

 is mostly ciliated, and it is composed of high and narrow 

 cells, some o£ which have basal nuclei, and apparently nerve 

 connections, so that they are very probably sensory cells. 

 A little way in from this border the epithelium is a good deal 

 higher, very regular, and close set, and the surfaces of these 

 extra high cells have a thick covering of uncertain nature 

 (fig. 6). In this way the height of the band is much 

 increased, and, as the bauds of successive leaflets are opposed 



