THE CELL AND PROTOPLASM. 



a t n. 



animal is composed of a single cell. These 

 organisms are usually of extremely minute size, 

 and they comprise most of 

 the so-called animalculse 

 which are found in water. 

 In such animals the differ- 

 ent parts of the cell are mod- 

 ified to perform different 

 functions. The different or- 

 gans appear within the cell, 

 and the cell is more complex 

 than the typical cell described. 

 Fig. 21 shows such a cell. 

 Such an animal possesses 

 several organs, but, since it 

 consists of a single mass of 

 protoplasm and a single nu- 

 cleus, it is still only a single 

 cell. In the multicellular or- 

 ganisms the organs of 

 the body are made up of 

 ceils, and the different 

 organs are produced 

 by a differentiation of 

 cells, but in the uni- 

 cellular organisms the 

 organs are the results 

 of the differentiation of 

 the parts of a single 

 cell. In the one case 

 there is a differentiation 

 of cells, and in the 

 other of the parts of a 



cell 



FIG. 21. A complex cell. It is c u u ' f ' V, 



an entire animal, but composed oUCn, in Dnei, IS the 



of only one cell. cell to whose activities 



6 



