170 THE FRESH-WATER MUSSEL [OH. VI 



shell. Its protrusion is effected between the ventral and 

 anterior margins of the valves, whence it may sometimes 

 be seen projecting as a tumid, yellowish-pink, fleshy mass, 

 for a distance of several inches. I have seen the foot 

 reflected over the shell of a large specimen to such an 

 extent as to cover almost the entire valve and to touch 

 the inhalant siphon with its apex. When the animal 

 wishes to move forward the foot gradually extends, 

 insinuating itself into the mud or into narrow crevices 

 between pebbles and stones in such a way as to secure 

 a firm hold by lateral pressure; the sides of the foot, 

 especially in the more ventral and anterior regions, then 

 swell so as to press even more firmly against the enclosing 

 mud, and at the same time by a convulsive heave the 

 shell rides forward on the foot, and then slightly down- 

 ward. During this movement of the shell very little, if 

 any, of the foot disappears within the shell ; in fact 

 the valves are at the moment forcibly drawn together so 

 as to pinch the foot and prevent the shell from slipping 

 down over it, or the foot itself from being drawn back 

 into the shell. Hence the effect, so long as the valves are 

 approximated, is that the shell is pulled evenly forward in 

 a horizontal direction : when the adductors relax and the 

 valves gape apart the shell drops down over the foot for a 

 slight distance. In this way the animal advances step by 

 step ; there is no steady forward gliding as in gastropods. 



At the close of each progressive movement the valves, 

 as already stated, separate slightly, but the mantle edges 

 are kept firmly applied to one another so that there is a 

 decided negative pressure within the shell cavity. This 



