THE BEHAVIOR OF AMCEBA 



At the same time the inward pressure is decreased in the region of 

 lowered tension, while elsewhere the pressure remains the same. Hence 

 the internal fluid of the drop is pressed out toward the region where the 

 film is weakened ; a current flows in the central part of the drop toward 

 this point. This current may produce a projection at the point of 

 lowered tension, provided the surface currents do not carry the fluid back 

 as fast as it is brought forward. 



It was long supposed that the movements of all sorts of Amoebae 

 were of this character. As a natural conclusion, it was commonly held 

 that locomotion and the formation of pseudopodia in Amoeba are due 

 to a local decrease in surface tension at the region of forward move- 

 ment. As our account shows, most Amoebae do not move at all as do 

 liquid drops whose movements are produced through changes in surface 

 tension. 1 Rolling movements with all currents forward cannot be pro- 

 duced experimentally through local changes in the surface tension of a 

 drop of fluid. It is necessary, therefore, to abandon the surface tension 

 theory for those Amoebae that move in the way shown in Fig. 6. If the 

 theory is still maintained for the Amoebae with backward currents, this 

 involves holding that the movements are due to fundamentally dif- 

 ferent causes in different Amoebae ; this is the view maintained by 

 Rhumbler (1905). 



While most Amoebae roll as they progress, different species differ greatly 

 in special features of their movements. The species of the verrucosa 

 type (Fig. 3) move slowly and change 

 form very little, not sending out 

 pseudopodia. Those of the Umax 

 type (Fig. 2) move more rapidly and 

 change form more frequently, but they 

 rarely send out pseudopodia. Finally, 

 in the proteus type (Fig. 1) the form 

 is excessively changeable, many pseu- 

 dopodia extending and retracting. 

 Many Amoebae show what might be 

 called specialized habits in their usual 

 movements. For example, Amoeba angulata and Amoeba velata usually 

 send forth at the anterior edge a pseudopodium which extends freely into 

 the water and waves back and forth, serving as a feeler or antenna 



Fig. 8. — Amwba velata, showing the 

 antennalike anterior pseudopodium pro- 

 jecting freely into the water. After 

 Penard (1902). 



1 According to Rhumbler (1905), such movements are most readily seen in a species of 

 Amoeba living parasitically in the intestine of the cockroach. Whether the currents on the 

 upper surface are actually backward, where the interior currents are forward, as is required 

 if the movements are to be explained by local decrease of surface tension, has not been 

 shown. 



