35 



addition to these there is another layer of mucous glands 

 (fig. 10, Mu. g. c.) situated more internally but not far 

 from the epidermal layer above described. These appear 

 to be similar to the mucus-secreting glands described by 

 Johnstone (4) as occurring on the foot of Cardium. The 

 glands consist of groups of cells aggregated together; 

 sometimes where a group is more distinct it can be seen 

 to consist of about 5-8 cells forming a kind of bulb. 

 From this clump of cells a long stalk arises which passes 

 to the surface and insinuates itself between the epithelial 

 cells ; it may divide into two or more branches just below 

 the epidermal layer. The stalk is non-tubular, and the 

 contours of the cells composing it cannot be distinguished. 

 The ground substance of the cells is finely granular, and 

 stains a peculiar grey-blue tint with methyl-blue-eosin. 



Under the epidermis there is a layer of connective 

 tissue comparatively free from muscle fibres, and the 

 rest of the foot is made up, as previously mentioned, of 

 connective tissue and muscles. Large blood spaces are 

 to be found scattered through the connective tissue and 

 connected at the base of the foot with the pedal artery 

 which enters it on the dorsal side ; the blood lacunae are 

 connected with others which pass over the visceral mass 

 to the dorsal extremity of the renal organ. 



There is also a very abundant nerve supply; the 

 pedal ganglia, as will be seen later, are not situated in 

 the foot. Two pedal nerves pass from these, and after 

 entering the foot break up into smaller bundles (fig. 8, 

 N. p.), which ramify amidst the connective tissue and 

 innervate the muscles. 



The Byssal groove which is seen on the ventral 

 surface of the foot, is a deep groove lined by ciliated ceFs, 

 and extending almost half way across the diameter of the 

 foot. In Pecten opercularis the foot is twisted so that the 



