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 Amoeba 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 Amoeba 

 proteus show that the protoplasm does not flow around the 



