192 Proceedings of Eoyal Society of Edinlurgh. [march 5, 
It moves about, seemingly without any definiteness in its move- 
ments. 
To give some idea of its rate of movement, it may be mentioned 
that in one trial, at the end of 43 minutes, it was 1 inch in a straight 
line from its starting point, although in its irregular movements it 
had actually traversed a much greater distance. 
The duration of movement in four specimens was from one to 
three days. The rate of movement of the current produced was 
about 1 inch in 3 minutes. 
Explanation of Direction of Movement . — The foot detached is 
constant in its direction, always moving forward in the direction 
of the tip, never backward in the direction of the base of cut 
surface. 
When examined under the microscope the cilia are plainly seen, 
and the direction of the ciliary current, as might be anticipated from 
the movement of the mass, is towards the base. At the tip there is 
a strong inward surface current, and on each side the marginal 
current passes backwards ; but if the cilia working at the posterior 
end of the grooved ventral surface are carefully examined, they are 
found to create a strong current forward. This forward current is 
too confined, however, to interfere with the general movement, unless 
perhaps to retard it a little; but its probable use will be afterwards 
pointed out in dealing with function. 
The direction of the currents on the attached foot may be very 
easily demonstrated by means of small pieces of sea-weed placed 
upon it. A tiny piece of sea-weed placed on the tip (ventral surface) 
travelled backwards to the attachment of the byssus, a measured 
distance of 1 inch, in 3 minutes. Afterwards it- crept much more 
slowly along the projecting portion of the abdomen. The rate, of 
course, will depend on the bulk of the body carried, for a larger 
piece of sea-weed placed on the tip travelled \ inch only in about 
1 hour 20 minutes, audit was carried towards the margin. Any- 
thing placed upon the ventral surface of the foot is sooner or later 
driven backward, and either thrust off at the side or carried towards 
the base. 
The reason for the direct straight forward course of the foot is 
now evident. The surface and marginal currents are towards the 
base, hence The direction of motion is towards the tip. 
