188 
ZOOLOGY: MAST AND ROOT 
OBSERVATIONS ON AMEBA FEEDING ON INFUSORIA. AND 
THEIR BEARING ON THE SURFACE-TENSION THEORY 
By S. O. Mast and F. M. Root 
ZOOLOGICAL LABORATORY. JOHNS HOPKINS UNIVERSITY 
Received by the Academy, February 25, 1916 
A considerable number of investigators hold that movement in Ameba 
is produced exclusively by changes in surface tension. Among the most 
prominent of these we may mention Biitschli (1892), Ryder (1894), Jen- 
sen (1905), and Verworn (1909). Many others, e.g., Rhumbler (1910) 
and McClendon (1911), hold that while surface tension may not be the 
only factor involved in the process in question, it certainly is one of the 
most important. The results of our observations do not support these 
contentions. 
Certain amebae at times feed almost exclusively on rotifers, at others 
they feed largely on paramecia. They capture the rotifers by flowing 
around the foot at the point of attachment to the substratum. After 
they have surrounded the foot they begin to flow out over the body. 
The rotifer responds by contracting and forcing the ameba back, after 
which it extends again and the ameba again begins to flow out over it, etc. 
In the meantime the foot begins to digest and gradually the rotifer 
weakens. Thus they continue sometimes for days before the rotifer is 
swallowed. The whole process is of such a nature that after observing it 
we were fully convinced that the force exerted by the amebae was far 
greater than could possibly be produced by changes in surface tension. 
The following observations prove this conviction to be correct with ref- 
erence to the process of feeding on Paramecium. 
When amebae are feeding on paramecia they assume a sort of mush- 
room shape with a serrate edge consisting of numerous short pseudopods. 
The paramecia tend to come to rest between and under these pseudo- 
pods by which they are usually surrounded, but sometimes the ends of 
the pseudopods approach each other before they are fully extended and 
cut the Paramecium in two. This process requires approximately only 
ten seconds. 
To cut a Paramecium in two with a fine glass fiber it requires a pres- 
sure of approximately 9 mgm. Consequently, if the pseudopods have 
the same cutting quality as the glass fiber and if their movement is due 
to a change of surface tension, it would require, to perform the work 
involved, a reduction in surface tension of at least 1118 dynes per 
centimeter at the tips of the pseudopods. But if the pseudopods fuse 
along the edges near the ends so as to form a ring around the para- 
