AMOEBA OF CLASS SARCODINA 91 



Behavior 



All of the activities of an animal which come in response to in- 

 ternal or external stimuli make up the "behavior." The activities 

 of the animal under discussion include the formation of pseudo- 

 podia, ingestion of food, locomotion, and others. Amoeba proteus 

 exhibits either positive or negative reactions to various stimuli. An 

 environmental change to which an animal reacts is known as a 

 stimulus, while the reaction of the animal is called the response. The 

 movements made by an animal in response to stimuli are called 

 tropisms. Amoeba exhibits all of the tropisms discussed in Chapter 

 IV. To physical contact, it responds positively if the impact is 

 gentle ; otherwise the response is negative. It responds negatively to 

 strong light and finds its optimum in a moderately reduced light. 

 When some part of the body surface of this animal comes quietly 

 into contact with food, there is a characteristic response. This part 

 of the protoplasm stops flowing while other parts continue, thus form- 

 ing a pocket around the particle of food. The edges of the pocket 

 fold in, meet, and join so as to enclose the object. This attraction to 

 food is likely a positive chemotropism. Amoeba reacts negatively 

 to concentrated salt, cane sugar, acetic acid, and many other chemi- 

 cals which have been tried. Amoebae have an optimum tempera- 

 ture range between 15° and 25° C. Temperatures approaching the 

 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 aai 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 

 amoehoid movement. It is so named from the perfect exemplification 

 of such activity by Amoeba. Locomotion is accomplished by the 

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



