79 



that of the ctenidial leaflets further in being modified 

 and specialised in different regions. These regions are 

 (a) sensory, (6) glandular, (c) ciliated, the former being 

 by far the largest in extent. The sensory region extends 

 over the greater part of the free lateral surface of the 

 leaflets. The glandular region is confined more or less 

 to the edges, and in particular to the lateral edges 

 (fig. 44, Osp. gl.). The ciliated area is a long narrow 

 strip extending along both sides of the leaflet against the 

 glandular edge and between it and the sensory area 

 (fig. 44, Osp. cil.). Thus in a section taken through 

 A-B (fig. 44), the glandular area is nearest B, that is 

 where the free lateral edge meets the mantle, this is 

 followed by the ciliated region, and then the remainder 

 is sensory. The sensory area is characterised as in the 

 lamellibranchiata by an absence of cilia, and a con- 

 siderable thickening of the epithelium. This thickening 

 is due to an increase in the length of the cells, which are 

 further modified by the possession of a yellow granular 

 pigment. Now the real structure of this important area 

 is not easily investigated. Sections well preserved and 

 fixed show that it is built up of a large number of 

 pigment-bearing cells, whose nuclei are apparently 

 arranged at different levels because one sees nuclei close 

 to the basal membrane, and from here to near the free 

 surface they are irregularly scattered (fig. 45, Osp. so.). 

 There is, however, a stratum close to the free margin of 

 the cells perfectly free from nuclei, and it is here that the 

 pigment is situated. 



Macerations of the osphradium explain the structure 

 instantly. The pigment-bearing cells are of the type 

 indicated in fig. 46, and all agree in possessing a terminal 

 plateau in which the pigment lies. From this plateau 

 the cell becomes constricted and fibre-like, with, however, 



