34 



wliicli is almost bifid has a very deep cavity, tlie dorsal 

 boundary wall of wliicli 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. h.). 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 

 groove. 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 bouuded 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 

 margin 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 

 cavity will be almost always found full of mucus. In 



