292 ESSENTIALS OF ZOOLOGY 



freezing point inactivate the animal, while temperatures above 30° 

 C. (86° F.) also retard their activities and may soon become fatal. 

 A weak electric current has an effect on the physical condition of 

 the protoplasm on the side nearest the cathode. The tendency is 

 toward the sol state here, hence the animal turns toward the cathode. 

 According to Jennings, who has done extensive research on be- 

 havior of Protozoa, these activities are ''comparable to the habits, 

 reflexes, and automatic activities of higher animals." He also feels 

 that Amoeba probably experiences pain, pleasure, hunger, desire, 

 and the other simple sensations. 



Amoeboid Movement and Locomotion 



The flowing or streaming of the protoplasm and extending the cell 

 in some direction by the formation of pseudopodia is usually called 

 amoeboid movement. It is so named from the perfect exemplification 

 of such activity by Amoeba. Locomotion is accomplished by the 



~" "^ Particle - "" 



Pseudopodtum 



Fig. 98. — Successive positions in the movements of an amoeba viewed from the 

 side. Notice the formation of new pseudopodia and the engulfing of the particle on 

 the surface. (Modified from photographs by Bellinger, 1906, Journal of Experi- 

 mental Zoology.) 



pseudopodia, and the process of their formation in most Amoebae. 

 Successive pseudopodia are formed in the moving Amoeba proteus as 

 it goes in a given direction. The pseudopodia are temporary loco- 

 motor structures. Most zoologists explain this movement as being 

 due to the contraction of the more viscous ectoplasm, particularly in 

 the ''posterior" region. This brings about a forward movement in 

 the more fluid endoplasm (plasmasol) which causes an outflow at 

 points where the ectoplasm is thinnest, or where surface tension is 

 lessened. As this plasmasol approaches the advancing tip of the 

 pseudopodium, it turns to the sides and changes to more solid endo- 

 plasm (plasmagel). This process continues, pushing the advancing 

 tip farther and farther forward. At the opposite side, the plasma- 

 gel continues to become plasmasol to provide for fluent material. 

 At the side of the animal away from the advancing pseudopodium, 

 the cell membrane (plasmalemma) moves upward and over the up- 



