SINGLE-CELLED ANIMALS AS ORGANISMS 171 



begins life again, as before. In this respect it resembles the 

 spore of a plant. 



The Cell as a Unit. In the daily life of a one-celled animal we 

 find the single cell performing all the general activities which we 

 shall later find the many-celled animal is able to perform. In the 

 amoeba no definite parts of the 

 cell appear to be set off to per 

 form certain functions ; but 

 any part of the cell can take in 

 food, can absorb oxygen, can 

 change the food into proto 

 plasm, and excrete the waste 

 material. The single cell is, in 

 fact, an organism able to carry 

 on the business of living almost 

 as effectually as a very com 

 plex animal. 



Complex One-celled Ani 

 mals. In the paramcecium 

 we find a single cell, but we 

 find certain parts of the cell 

 having certain definite func 

 tions : the cilia are used for 

 locomotion ; a definite part of 

 the cell takes in food, while the 

 waste passes out at another 

 definite spot. In another one- 

 celled animal called vorticella, 



Vorticella. e, gullet ; n, nucleus ; cv, con- 

 part of the Cell has become tractile vacuole ; a, axis ; s, sheath ; fv, 

 elongated and is Contractile. food vacuole. (From Herrick's General 



Zoology.) 



By this stalk the little animal 



is fastened to a water plant or other object. The stalk may be said 

 to act like a muscle fiber, as its sole function seems to be move 

 ment ; the cilia are located at one end of the cell and serve to 

 create a current of water which will bring food particles to the 

 mouth. Here we have several parts of the cell, each doing a dif 

 ferent kind of work. This is known as physiological division of labor. 



