90 



BIOLOGY 



UNICELLULAR AND MULTICELLULAR ORGANISMS 

 All of the organisms thus far studied have been made up 

 of single cells, each cell being independent and capable of 

 carrying on all life processes within itself, although many of them 

 are quite complex, having several organs and much variety; 

 see Fig. 38. In contrast to these unicellular organisms we 

 shall find organisms made up of large numbers of cells (multi- 

 cellular organisms). All of the larger and higher animals 

 and plants in the world are made up of great numbers 

 of cells, each having the same general structure as the uni- 

 cellular organisms we have already studied. These larger 

 organisms begin their life as single cells and become multi- 

 cellular by the division of their cells into many parts. 

 There is no doubt that the ~ multicellular organisms of the 



world must have been de- 

 rived originally from the 

 unicellular organisms. 



Intermediate Types. 

 While the organisms de- 

 scribed in the last chapter 

 are called unicellular, there 

 are some of them to which 

 this term cannot be ap- 

 plied with strict accuracy. 

 Pandorina (Fig. 28), for 

 example, consists of a 

 group of sixteen cells at- 

 tached in a spherical, 

 gelatinous mass. Each of 

 these masses of sixteen 

 cells has been derived 

 from a single cell by divi- 

 sion. It is a question 



whether this organism should be called unicellular or multi- 

 cellular. It is certainly made up of more than one cell; but 



FIG. 38. BURSARIA. ONE OF THE LARG- 

 EST AND MOST COMPLICATED OF THE 

 SINGLE-CELLED ANIMALS 



/, food; 

 m, mouth; 



mb, membranella; 

 mac, macronucleus. 



