8 GILBERT C. BOURNE. 



section passes through the more distal part of the ridge, and 

 the ciliated canal is seen to be smaller and situated near the 

 dorsal side of the ridge. A little further on it ends blindly. 

 As the figures show, the ridge is made up of a sheath or 

 cortex of elongated, fusiform cells, which pass nearly trans- 

 versely routid the periphery of the ridge, and a medulla of 

 large, closely packed ovoid or fusiform cells having lai-ge nuclei 

 and granular cell contents. The cells abutting on the lumen 

 of the ciliated canal are usually larger and more granular 

 than those more peripherally situated, and their histological 

 characters leave little doubt that they are secretory. It is 

 noticeable that there are very few if any glandular cells 

 interspersed among the columnar ciliated cells of the filament, 

 and the glandular ridge appears to have taken over the 

 secretory functions, and to replace the secretory cells scattered 

 over the surface of gill-filaments of other Mollusca. 'J'he 

 extreme specialisation exhibited by the formation of a closed 

 canal into which the secretory cells discharge their products 

 is certainly a remarkable feature in Incisura. 



The central blood-channels of the filaments, as may clearly 

 be seen in the figures, are elongate-oval in shape, and their 

 walls are strengthened, for about half their extent, by 

 flattened, chitinous, skeletal bars, which, as in other molluscs, 

 may be traced to the proximal end of each filament, where 

 they diverge from one anothei*, and curve round to run up 

 in the walls of the blood-spaces of the adjacent filaments 

 (fig. 19). As M. F. Woodward (19) has shown that in 

 Pleurotomaria these skeletal bars run along the dorsal edges 

 of the gill-filaments, whereas in Nucula they run along the 

 ventral edges, it is of some interest to determine the position 

 of these bars in Incisura, which is usually reckoned as 

 belonging with the genera Scissurella and Schismope to the 

 Pleurotomariidae. It is clear from an inspection of fig. 17, 

 representing a transverse section through the posterior gill- 

 filaments of the left ctenidium, which, as explained above, 

 are turned upside down, that the skeletal bars lie on the 

 dorsal sides of the filaments, and the same thing can be 



