66 PROTOZOA 



outer motionless part of the endoplasm; and the plasmasol, which is the 

 central moving portion of the endoplasm. This is shown in Fig. 20, which 

 represents a simple type with one pseudopodium to which also the follow- 

 ing description applies. The plasmalemma is quite permanent and is 

 stationary over that part of the surface in contact with the substratum, 

 to which it adheres. At the side of the animal away from the advancing 

 pseudopodium, the plasmalemma is being carried upward and over upon 

 the upper surface ; on this surface it is being carried forward ; at the tip of 

 the advancing pseudopodium it is moving downward; and it is laid down 

 against the substratum in advance of that portion which is temporarily 

 stationary and of which it now becomes a part. Through the center of 

 the animal is a forwardly directed flow of plasmasol, which as it 

 approaches the advancing tip turns to the side and becomes plasmagel. 



> 



b a 



Fig. 20. — Diagrammatic representation of a simple ameba, such as Amoeba verrucosa 

 Ehrenberg, viewed from the side and moving by the formation of a single pseudopodium. 

 The arrows along the surface show the direction of movement of the plasmalemma, which is 

 stationary against the substratum from a to b. The arrows within the body show the 

 direction of flow of the plasmasol from the area of solution at the temporary posterior end to 

 the area of gelation near the temporary anterior end. The arrow above the figure shows 

 the direction in which the animal is moving. 



The onflow of plasmasol continues, serving to push this tip forward more 

 and more. At the other side, the plasmagel is continually becoming 

 plasmasol and thus providing the material for the continued flow. The 

 forward movement of the ameba is accompanied by a continuous gelation 

 and decrease in volume of the protoplasm toward the tip of the develop- 

 ing pseudopodium and a continuous solution and increase in volume of 

 that at the opposite side. In the case of an ameba with several pseudo- 

 podia, one or more may be developing in the manner described above, 

 while others may be disappearing. The latter will exhibit solution toward 

 the tip and a flow of plasmasol back through the center into the main 

 body. The elastic plasmalemma will show extension or contraction in 

 different directions as it becomes adjusted to the changing outhne. There 

 is a curious resemblance in the manner of functioning of the plasmalemma 

 to the continuous metalhc belt which forms the tread in a caterpillar 

 tractor. 



97. Behavior. — An ameba exhibits reactions to various stimuli. To 

 contact it responds positively if the contact is a gentle one but negatively 



