34 



COLLEGE ZOOLOGY 



nected with locomotion and those resulting from external and 

 internal stimuli. 



Locomotion. — Ameba moves from place to place by means 

 of finger-like protrusions of the body, known as pseudopodia 

 (Fig. 9, j). A pseudopodium is formed in the following manner. 

 The ectoplasm bulges out and enlarges until a blunt projection 

 is produced; the endoplasm then flows into it. The result is 

 a movement of the entire animal in the direction of the pseudo- 

 podium. If more than one are formed at the same time, there 

 occurs a struggle for supremacy until finally one survives while 

 the others flow back and gradually disappear. Ameba moves, 

 therefore, by thrusting out pseudopodia and then flowing into 

 them. 



There are three principal theories which attempt to explain 

 the formation of pseudopodia. (i) The adherence theory holds 

 that the pseudopodium adheres on one side more strongly than 

 on the others, and that the entire animal, therefore, moves to- 

 ward the adhering side. (2) The surface tension theory maintains 

 that local changes in the surface tension cause the currents which 

 initiate movement. (3) According to the contractile theory, 

 Ameba moves by means of a contractile substance in the follow- 

 ing manner. In advancing the Ameba " extend the anterior 

 end free in the water and attach it at or near the tip and then 

 contract. At the same time the posterior end is contracting 

 and the substance thus pushed and pulled forward goes to form 

 the new anterior end (Fig. 12, A, B). ... In other cases 

 the anterior end is lifted free and then curves down to the sub- 

 stratum and attaches, forming a long loop. The posterior end 

 is then released, and the substance, flows over to the anterior 

 end. At the same time another anterior end is extended (Fig. 

 12, C)." 



There are various methods of imitating the movements of Ameba 

 by means of inorganic substances. One of these is as follows: 

 A large drop of mercury is placed in a flat-bottomed watch 

 glass and covered with 10 per cent nitric acid. A piece of 



