THE CELL, THE LIVING UNIT 3 



animal performs exactly the same processes as the 

 complicated one. All animals must breathe, must 

 feed, must excrete waste matters, and must increase 

 in numbers or reproduce themselves. The single, 

 tiny cell that constitutes the simplest animal can 

 do all these things. Its protoplasm can absorb 

 oxygen from the surroundings in which it lives that 

 is, it can breathe. It can take in and digest food, 

 building it into new, living protoplasm. The waste 

 products, both of its food and those produced by its 

 own activity, are expelled from the body of the tiny 

 organism. When need arises, it can divide into two 

 portions each resembling itself, and so form two new 

 individuals; or in some cases, hundreds of little 

 organisms may be produced by its division, each 

 growing like the parent and each capable of living 

 like its progenitor. 



Again, among some of these one-celled animals 

 clear differences, analogous to those observed in 

 higher animals, are found. Sex is present, even in 

 some of these very early and primitive forms of life, 

 and from the union of male and female one or more 

 minute progeny arise. These, being invigorated by 

 the union of their parents, carry on the race with 

 greater capacity than do those organisms produced 

 without the intervention of sex. The single-celled 

 animals are very numerous so numerous, in fact, 

 that they form a large group in the animal kingdom 

 known as the Protozoa, or primitive living animals. 



The situations in which the Protozoa are found 

 depend largely on their mode of life. As in higher 

 communities, some of the Protozoa live free, in- 



