ONE-CELLED ORGANISMS 27 



amoeba, the body is likewise of microscopic proportions, 

 but retains a definite shape owing to a permanent cell 

 membrane. The body is thickly covered by a coat of 

 delicate protoplasmic hairs or cilia whose rhythmical 

 beating drives the organism from place to place. At one 

 point the cell-wall is perforated, and through this mouth 

 opening food particles are driven by special cilia into the 

 interior. In other respects the life processes resemble 

 those of the amoeba. It is to be noted, however, that the 

 paramoecium possesses definite organs of locomotion and 

 a mouth; and, as will appear more clearly in a succeed- 

 ing paragraph, this constitutes a real advance beyond the 

 amoeba. 



Importance of One-Celled Organisms. — While 

 these two examples are insufiicient to illustrate the be- 

 wildering diversity of structure of the thousands of named 

 species of these one-celled animals and plants, they may 

 be used to emphasize the fact that all lead successful 

 lives. Ministering to their needs, avoiding enemies, and 

 leaving offspring to take their place, they exist and have 

 persisted for thousands of years, and this constitutes a 

 successful life. Also in point of numbers of individuals 

 they far outrank all other living things. In many re- 

 gions certain aquatic forms settle to the bottom, and 

 their skeletons of lime or flint form deposits of great ex- 

 tent and thickness. Chalk, tripoli earth, and other simi- 

 lar substances are ancient accumulations that are of high 

 economic importance. The oil enclosed in the bodies 

 of the minute aquatic diatom plants have formed, it is 

 believed, the great b^ds of natural oil in the western part 

 of North America. Finally, there are many species that 

 derive their food supply from other organisms, and as 

 parasites produce diseases that rank among the most 

 serious scourges known to man. 



Colonial Forms. — Among the greater number of one- 

 celled organisms the division of the body results in two 

 offspring that separate and thenceforth lead independent 



