366 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
which is almost bifid has a very deep cavity, the dorsal 
boundary wall of which extends further distally than the 
ventral, which is notched. 
Two sides of the foot can be distinguished, the dorsal 
and the ventral, the latter has a groove running 
longitudinally along its surface for about half of its 
length (fig. 7, P.6.). This is the byssal groove and 
communicates with the byssal gland. 
The deep cavity of the end of the foot is continued 
down the centre until it almost reaches, if it does not 
communicate with, the cavity of the byssal gland and 
sroove. The foot is very contractile, and in fixed 
specimens is usually much contracted and wrinkled; it 
does not contain any extensions of the viscera, and the 
greater part of its bulk is composed of muscle fibres 
running in various directions in a groundwork of 
connective tissue. It is bounded by the usual epidermal 
layer of epithelial cells, which are columnar, the depth 
being about three times the width. These cells are 
ciliated over the whole outer surface, and even extend 
into the deep cavity of the end of the foot. These ciliated 
cells are very fine objects for showing the striated cell 
marein seen in ciliated epithelium. 
The epidermis lining the cavity of the foot differs, 
however, from that on the outer surfaces in that the 
epithelial cells are compressed in the middle part of their 
length, so that they are somewhat hour-glass shaped and 
have interposed between them many mucous glands (fig. 
10, Mu.g.). In these, nuclei are indistinguishable, but 
from the size and shape it is extremely probable that 
these glands are unicellular. In places, in sections, the 
mucus can be seen emerging from between the epithelial 
cells, and if the foot of living specimens is examined the 
eavity will be almost always found full of mucus, In 
