CHAPTER XIII 

 MOVEMENT IN ANIMALS 



Movement in Protozoa. — The Protozoa, as we name 

 the one-celled animals, live in water where they either 

 move about freely or are attached by some part of their 

 protoplasm to some solid object. As we stated in an 

 earlier chapter (III), there are very many different 

 kinds of Protozoa which differ greatly in appearance as 

 well as habits. We can distinguish among all of them 

 four types of movement which serve the entire cell, as 

 distinguished from movements of protoplasm within 

 the cell. 



Amceboid Motion. — The first of these is the charac- 

 teristic form of motion of the very simple organism known 

 as the amoeba (Fig. 2), and this kind of motion is, there- 

 fore, called amoeboid movement. The motion consists of 

 •a thrusting out of the protoplasm in some direction with 

 an equivalent drawing in of other parts, so that the 

 volume of the cell is not changed, although the outline 

 of the cell is continually changing. The movement is not 

 rapid. An amoeba usually has to be watched carefully 

 under the microscope to make out whether it is moving 

 or not. These amoeboid movements appear to be purely 

 random, yet if an amoeba is observed for a period of 

 time it is usually seen that it makes progress in one direc- 

 tion, so that out-thrustings in that direction must have 

 been on the whole more frequent than in any other. We 

 ^PPly the expression amoeboid movement to any cell 

 motions in which the protoplasm is thrust out bodily. 



Ciliary Motion. — The second kind of motion is 

 known as ciliary. It receives this name from the fact 



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