16 



desiccation, to which it is specialh^ liable, iu its habitat 

 far up the tidal zone, and because of the greater ratio of 

 surface to volume in smaller than in larg^er forms. 



The foot ffig'. Sa) is covered externally by a laver of 

 columnar epithelium, which possesses more or less of a 

 cuticle, with o'ol)let cells here and there. The cells on the 

 side of the foot are small, but increase in height towards 

 the ventral surface, where they are fairly long and regular, 

 with nuclei usually near the base. The ventral covering, 

 as might be expected from the amount of wear to which 

 it is subjc'cted, seems to be shed frequently. The 

 epithelium of the sides of the foot is throAm into a 

 number of small ridges, which, in very young specimens, 

 are found nearly up to the mantle attachment, but in 

 older animals are mainly restricted to the part near the 

 ventral edge. Internal to the epithelium of the sides of 

 the foot is a deeply staining layer with nuclei, which is 

 apparently a sub-epithelial nerve plexus, and from it 

 there proceed outwards narrow deeply staining cells 

 which are probably sensory. Beneath this covering tissue 

 is a dermis, an irregular network of connective tissue 

 with blood spaces which are more numerous internally. 

 The dermis is thin at the sides of the foot. Along the 

 anterior border of the foot there occur several small glands 

 opening near the edge of this organ. 



Internal to the dermis are found the muscle bundles 

 wliich make up the main mass of the foot. Most of these 

 bundles are continuous with those of the shell muscle, 

 and, in fact, the horse-shoe impression might be spoken 

 of as tlie area of attachment of the muscle fibres of the 

 foot. Of these bundles the outer ones proceed downwards 

 in a steeply oblique direction, while the inner ones 

 cnvve in and liecome tlie slightly oblique bundles which 

 fdini ih(^ dorsal part of the foot, lietween these bundles 



