166 G. H. PARKER 
longitudinal muscles and can be best pictured by reference to the 
accompanying diagrams. These diagrams represent steps in the 
passage of a concave wave over the foot of a snail from an anterior 
position to a posterior one (left to right in the diagram) whereby 
the point x is temporarily released from full adhesion to the mu- 
cous surface, moved forward, and brought to full adhesion again. 
The point x is supposed to be associated with a particular longi- 
tudinal muscle fiber, number 2, through whose action it is moved. 
In A, this fiber is shown in its relaxed condition with the wave 
approaching. In B, the wave has released the point 2 from full 
adhesion. In C, fiber 2 has contracted and since the posterior 
end of it is over a released part of the foot and the anterior end 
over a fixed part, the posterior end with the underlying point z 
has been moved anteriorly. In D, the fiber remains contracted 
and the point x has come again to adhere to the substrate. In £, 
the wave has reached the next longitudinal fibre anterior, number 
3,which has contracted and drawn out the relaxing fiber, number 2, 
to its original length and position in reference to point x. The 
contraction of each longitudinal fibre then serves two purposes: 
it moves the foot forward as the releasing wave passes over the 
region and it extends the relaxing posterior fiber. In this way each 
