296 MOVEMENT IK LOWBK ANIMALS 



main there unless it can produce such changes in its com- 

 position as to make it lighter than the water it displaces. 

 Certain one-celled animals have the power to accumulate 

 carbon dioxide within the cell bodies to such an extent as 

 to decrease their specific gravity. Thus, the body be- 

 comes lighter than the liquid it displaces, and the buoyant 

 force of the water floats it upward. Further, they can 

 discharge this gas, increase their specific gravity, and sink, 

 as they have become heavier than the liquid displaced. 

 In this way many one-celled, water-dwelling animals are 

 able to change their position without the aid or use of 

 special organs of locomotion. In the higher animals, 



Fig. 127 — Swim bladder of the flsh. 



the swim bladder of the fish and the connection of the 

 Itmgs of the bird with cavities of the body, by means of 

 which air can be introduced into the body, are apparently 

 parallel cases, but it must be noted that in these cases 

 the decrease in body specific gravity is due to the action 

 of special muscles and not to changes in the composition 

 of the protoplasm of the cells. The result, however, is 

 the same. 



Locomotion due to secretion. — There are certain forms 

 among the one-celled animals and plants that are able to 

 creep over surfaces without contractile movements, or any 

 apparent organs of locomotion. Examination shows that 



