17 



groove. The rest of the epidermis of the viscero-pedal 

 mass consists of short columnar, non-ciliated cells, with a 

 continuous thick cuticle. 



As on the internal surface of the mantle edge, this 

 ciliated portion of the foot is also an area provided with 

 unicellular mucus-secreting glands. These form a con- 

 tinuous dense layer beneath the ciliated epidermis, and are 

 imbedded in the loose muscular sheath of this part of the 

 foot. Each of these glands (figs. 21 and 22, PL IV.) consists 

 of either a single cell or a group of from 2 to 6 cells aggre- 

 gated together. Single-celled glands are uncommon, and 

 are only found near the posterior limit of the glandular 

 area. Towards the tip of the foot they are more complex ; 

 the largest groups measure about 0*3 mm., the stalks 

 being about half that length. Such a complex gland con- 

 sists of a group of cells forming a bulb with a long stalk. 

 The limits of the separate cells are not always clearly 

 distinguishable. The cell bodies consist of a reticulum, 

 some of the bars of which are rather coarse ; these are, 

 however, continuous with a very fine meshwork, which 

 apparently makes up the cell substance. This reticulum 

 is continuous from cell to cell. Nuclei are not evident, 

 but in every cell there is a nodal point at the intersection of 

 several of the coarser bars of the reticulum ; reticulum 

 and nodal points, and the finest ground substance, stain a 

 intense blue with haematoxylin. The stalk is non-tubular, 

 has the same structure as the bulb, and breaks up at its 

 free end into a small number of branches which penetrate 

 between the epidermal cells and form little knobs among 

 the cilia on the surface of the foot. The structure and 

 staining reaction of these bodies, coupled with the habit 

 which the very young cockle has of pulling itself along a 

 smooth surface by causing the tip of its foot to adhere to 



