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 

 amceba 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 paramoecium 

 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, 

 part of the cell has become 

 elongated and is contractile. 

 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. 



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

 tractile vacuole ; a, axis ; s, sheath ; Jv, 

 food vacuole. (From Herrick's General 

 Zoology.) 



