SEA-FISHERIES LABORATORY. 367 
addition to these there is another layer of mucous glands 
(fig. 10, MWu.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 
eonnected 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, 
NV. 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 cells, 
and extending almost half way across the diameter of the 
foot. ‘In Pecten opercularis the foot is twisted so that the 
