THE PROTOZOA 121 



or long and thread-like. Several pseudopodia may be put simul- 

 taneously in A. proteus, and so the animal in life presents a very 

 characteristic motile appearance, and, further, its mode of movement 

 is characteristic of a number of other forms of protozoa, and is 

 described as amoeboid. Thus Amceba exhibits very completely 

 the power of motility. 



If we watch a living specimen we shall see that it moves freely 

 from place to place without any apparent cause. This form of 

 movement in ourselves or the higher animals we should term 

 voluntary, but in Amceba it is called automatic or spontaneous. A 

 few simple experiments will show that another sort of movement is 

 also possible. For example, if the slide is jarred, or certain chemicals 

 are acbied, the animal will withdraw its pseudopodia, or perhaps 

 move away from the chemical, thus acting in response to an external 

 stimulus. Such movements clearly indicating the power of irrita- 

 bility or sensitivity are termed induced as opposed to spontaneous. 

 It is, of course, impossible to draw a hard-and-fast distinction 

 between them, for we have no means of knowing what stimuli are 

 playing on the animal, and so we cannot be certain what are spon- 

 taneous movements. Some authors go so far as to claim that all 

 movements of Amceba can be explained in terms of environmental 

 stimuli. Changes in temperature undoubtedly play a great part 

 in determining the animal's activities, as we can easily test by con- 

 trolling the temperature carefully. At 25 C. Amceba exhibits its 

 greatest activity, and so this is termed the optimum temperature ; 

 above and below this it becomes more and more sluggish, movement 

 finally ceasing altogether when the temperature gets very low, near 

 freezing-point, or reaches 35 C. At 40 C. the protoplasm undergoes 

 striking changes and, as we say, coagulates, in the same way as the 

 white of an egg on cooking, and the animal is thereby killed. 



Feeding in Amceba is a very simple process, the animal flows 

 along until it encounters a particle of food, a piece of organic debris 

 or a diatom, etc. It goes straight on and engulfs or ingests the piece 

 which is passed through the ectoplasm into the endoplasm, and 

 there has formed around it a food vacuole. It is necessary that the 

 food should be organic matter of some sort, for Amceba is holozoic 

 in its feeding and cannot utilise inorganic substances. Within the 

 food vacuole the organic matter is digested until nothing but an 

 indigestible residue remains. It is not yet possible to examine the 

 substances in the fluid of the vacuoles, although it appears to be 

 acid to start with, and later becomes alkaline. Therefore, by analogy 

 we should expect it to contain enzymes, which are similar in action 

 to those in the digestive juices of the frog, since the composition of 

 the organic food matter is similar in the two animals. When it is 



