15 



as stated above. This glossy ventral layer is broadest 

 under the posterior margin (Text-fig. 2a, a). The 

 operculum is composed of three layers, a very delicate 

 superior layer, a median layer of considerable thickness 

 which is itself formed of numerous laminae arranged 

 at an acute angle to the plane of the operculum, 

 and a third and most ventral layer, the glossy one 

 described above. The thin superior layer is formed by a 

 pad of cells situated in a cleft below the anterior lip of 

 the opercular disc. The middle layer is secreted by the 

 cells of the disc at the margin of the region of attachment, 

 and chiefly anteriorly. As a matter of fact, the 

 epithelium of the foot is perfectly continuous underneath 

 the operculum, and it is by means of these cells that the 

 muscle fibres are attached to the operculum. The most 

 ventral layer of all is produced by the cells of the 

 opercular lip. It will be noticed that this lip is much 

 deeper and more obvious altogether jjosteriorly where the 

 glossy layer is widest and best developed. 



It is possible without more than decalcification to cut 

 sections through foot and operculum. These will show 

 quite easily the positions of the various secreting cells. 

 The secreting cells are characterised by their great depth. 

 They are narrow and about four times as deep as the other 

 epithelial cells of the foot in the vicinity. 



The pedal groove (PL II, fig. 9, Fed. gl. ; fig. 10, 

 Fed. gr.) is a deep incision running transversely across 

 the anterior margin of the foot. It appears quite early 

 in the larva, and is relatively very large at this period. 

 In transverse sections of the groove (PL II, fig. 10), or 

 longitudinal sections of the foot, a region round the base 

 of the groove can be seen with the unaided eye to be 

 different from the rest of the foot. Sections stained with 

 methyl-blue-eosin are very characteristic and make 



