440 EVOLUTION OF ANIMALS Part V 



and addition that goes on through its early hfc. Assimilation of food is its 

 essential preliminary. The extent of growth is determined by heredity and by 

 surrounding conditions, regulators that are as effective for an ameba as for a 

 horse. 



Reproduction. Amebas reproduce by division into two approximately equal 

 parts and by mitotic division of the nucleus. At a temperature of 24° C. the 

 process takes about half an hour. There is no real metamorphosis; an ameba 

 that has just come into existence by division looks like any other one of its 

 kind only smaller. 



Reactions to Stimuli. In their natural surroundings amebas are touching 

 something, are resting, or moving upon water soaked and decaying leaves. One 

 ameba described by H. S. Jennings touched the end of an algal filament, after 

 which a pseudopodium was extended along each side of the filament. Then 

 the protoplasm on one side stopped flowing and the filament was avoided 

 as part of the current was reversed and turned into another direction. If an 

 ameba is touched with a glass rod it behaves the same way. Reactions to con- 

 tact are not all negative, however. If an ameba comes in touch with a surface, 

 while it is still suspended in the water, it immediately spreads itself as a cat 

 landing from a jump will spread its toes to contact the ground (Fig. 10.1 ). In 

 general, amebas react positively to gravity; they creep on the bottom of a dish, 

 or on the mucky bottom of a pool, a contrast to the usual open water swim- 

 ming of paramecia. If salt solution from a very fine capillary tube diffuses 

 against the side of an ameba, the part affected will contract and the proto- 

 plasmic currents will start in another direction. 



Amebas are no more responsive in one part of the body than in any other 

 to touch, light, or other stimuli. In general, if light interferes with their 

 activities, they will move away from it. If it is suddenly thrown on them when 

 they are feeding on a filament of alga, they will stop and even squeeze out 

 bits of alga that were already ingested. Over-stimulation by light makes an 

 ameba refuse food as interference with equilibrium and other senses makes 

 some persons lose their appetite. 



Pursuit of Prey. In general, amebas draw away from things which would 

 be harmful to them and toward those that are beneficial. Most of their re- 

 sponses are due to direct physical or chemical stimuli from the environment. 

 Yet H. S. Jennings was not wholly able to analyze pursuits of one ameba by 

 another, although he observed them several times and devoted years of study 

 to the behavior of protozoans. One such pursuit and capture is described in 

 the figure and legend (Fig. 21.12). The captor (ameba c) pursued its 

 prey (ameba b) with great persistence. The climax came at numbers 11 and 

 12 when the captive (b) escaped completely out of contact with its captor 

 (c) yet the latter continued the pursuit and repeated the performance. Does 

 this not suggest that the ameba depends upon a primitive kind of memory? 



