10 



PHYSIOLOGY 



This independent movement from place to place is 



merely a mechanical response to some form of motion 

 within the cell. On the surface of 

 some of these animals are tiny, hair- 

 like projections called cilia (Fig. 2). 

 The protoplasm inside moves in such 

 a way that the cilia wave to and fro ; 

 the animal then moves through the 

 water like a boat propelled by oars. 

 Still others have a tail-like projection 

 which becomes fastened to something 

 in the water (Fig. 3). Within the 

 tail is a thread of protoplasm at- 

 tached alternately first on one side 



This thread 

 contracts and the animal is pulled 



away from anything that may happen 



to touch it. Movement in response 



to an outside stimulus, or irritation, 



is called irritability. 



Assimilation. — Another thing that 



attracts the attention as we watch 



these animals in the drop of water, is 



that some are larger than others of 



the same kind, just as cats are larger 



than kittens, and men are larger than 



boys. We may infer from this dif- 

 ference in size that they have the 



power of growth, or the power to 



add new protoplasm to their bodies, and that in order to 



get the materials for the manufacture of protoplasm, 



Fig. 2. — Paramecium 



showing cilia. (From and then on the other. 



McMurrich.) 



Fig. 3.— Vorticella 

 showing stalk of con- 

 tractile protoplasm. 

 (From McMurrich.) 



