CAUSES OF LOCOMOTION 295 



in oiir present discussion we are more directly concerned 

 with locomotion. 



Causes of locomotion in animals. — The locomotion of 

 all animal forms can usually be explained as due to one 

 of four causes : 



(a) A change of position due to the action of some force 

 outside themselves, such as currents of water or air. This 

 sort of movement is, of course, passive movement as far 

 as the animals are themselves concerned, since they talie 

 no part in its production. 



(b) A change of position due to change in the ceU com- 

 position, as a result of which there is a gain or loss in 

 specific gravity. Such change allows the buoyant force 

 of the water to move the body up or down. This is not 

 a passive movement, since the changes in composition 

 determine the action of the buoyant force. 



(c) A change in position due to active secretion on the 

 part of the cells composing part or all of the animal, and 

 an accumulation of this secretion in such amounts and 

 in such a manner as to force the body forward. 



(d) A change in position due to expansions and con- 

 tractions of part or all of the cell bodies composing the 

 animal, these movements being so arranged as to produce 

 a movement of the whole animal. 



The first cause needs little discussion, as it is due to no 

 physiological action 6i the animals affected. The other 

 three need some explanation to make their action clear. 



Locomotion due to changes in specific gravity. — Changes 

 of position of this character are usually confined to water- 

 dwelling forms. Protoplasm is slightly heavier than 

 water, and an animal, therefore, which rests upon the 

 bottom and has no apparatus for raising itself must re- 



