728 Proceedings of Royal Society of Edinburgh. [sess. 
power which enabled similar pieces of the sea-mussel to roam about 
in all directions. 
It will be observed that the mantle-lobe, in whole or in part, 
moved with its outer or non-ciliated surface uppermost. 
IV. Foot. — The foot is dark grey in colour and keel-shaped. 
It is capable of considerable expansion, and possesses great flexi- 
bility. Within the shell it is wedge-shaped, but when protruded 
it may enlarge to almost the size of the shell, 2J inches from a 
shell measuring 2J inches in length. Particles placed on the free 
ventral margin are carried inwards towards the body and then 
posteriorly. Although the entire detached foot does not move, 
the ventral margin is highly motile. A small strip from the 
anterior end moved forward 1J inch in 5 minutes. A piece 
detached from the central portion of the margin moved and 
wriggled about in various directions. Both ends were very sensi- 
tive, and in its contortions it travelled all over the plate. 
A strip from the whole margin moved irregularly over the plate, 
covering 7J inches in 7J hours. Every part of the ventral margin — 
anterior, middle, and posterior — was seen under the microscope to 
be richly ciliated. 
The anterior end of the foot moved in the direction of the free 
margin, rotating right-handed, 1 inch forward and f inch to the 
left. It would follow from the ciliary current being inward and 
posterior on the foot, that it would, when detached, move in the 
opposite direction, or towards the free margin. It is probable that 
the direction, when necessary, can be reversed, as indicated in the 
forward and backward movement mentioned above. And this 
applies not only to the palps and foot, but possibly to the gills 
and mantle-lobes ; for Tryon, in his Structural and Systematic 
Gonchology , mentions a common case of the “ inhalent ” becoming 
an “ exhalent ” current : — “ If an Anodonta be placed in a vessel of 
water into which some fine sand is introduced, the particles will be 
seen entering the incurrent siphon, and repelled from the orifice of 
the excurrent one; but after the animal has had enough of the 
unpalatable and irritating food, it will close its valves, forcing out 
the water, and with it the sand.” 
