68 THE BEHAVIOR OF PROTOZOA 
for the withdrawal. Sometimes pseudopods move back and 
forth as if they possessed a certain degree of flexibility and 
rigidity. The rolling motion of certain species offers another 
difficulty, so it cannot be said that, at present, the surface 
tension theory suffices to explain the movements of the 
organism. That it may play a part in the process is not 
improbable. It must not be forgotten that there are many 
surfaces within the mere outer layer, such as alveolar walls, 
etc., where surface tension may be a potent agent. In 
forms a little higher than Amceba we meet with a more or 
less fibrillar ectoplasm whose contraction occurs much as 
in an ordinary muscle fiber. It is not improbable that the 
fundamental features of contraction in the specialized ecto- 
plasm of flagellates and infusorians are the same as in the 
unspecialized ectoplasm of the rhizopods on the one hand 
and in the more highly specialized muscular tissue of higher 
animals on the other. The cause of muscular contraction 
is one of the most obscure problems of physiology and until 
it is solved we shall probably be unable to explain the mech- 
anism of the movements of the simplest animals. 
In taking food Amoeba has been described as flowing 
around an object and engulfing it in its endoplasm where it 
undergoes digestion. Surface tension has been supposed 
to play a part in the process. A fine splinter of glass brought 
against a drop of water will be quickly drawn in through the 
contraction of the surface film. A drop of chloroform. 
may be made to draw in a glass splinter covered with 
shellac; after the shellac is dissolved the glass splinter will 
be extruded, through the force of surface tension, as from a 
drop of mercury, thus simulating both the ingestion of food 
and the defecation of the undigested residue (Rhumbler). 
The observations of Jennings on food taking in Ameba 
proteus show that the protoplasm does not flow around the 
