120 GENERAL BIOLOGY 



The frog is one of the higher organisms made up of organs, which 

 in turn are made up of tissues consisting of sheets of similar functioning 

 cells, 



In the frog each group of tissues has a definite work to perform, 

 i. e., the eye only sees and the ear only hears, the bones only support, 

 and the heart only pumps blood. 



This specialization in the work of an organ is known as a division 

 of labor. 



There are hundreds of thousands of animals so small that they can- 

 not be seen with the naked eye, many of which are composed of only a 

 single cell. 



As they have only one cell they can have no tissues and conse- 

 quently no organs. But if they are living things they must possess the 

 four characteristics which distinguish living matter. 



They do have these four distinguishing characteristics. Conse- 

 quently the single-celled animal is as truly a living organism as is the 

 frog. 



But, as there are no organs and no tissues, the protoplasm in this 

 single cell must be able to do all the different kinds of work that the 

 .different organs in the frog do. 



Therefore, in one sense of the word, the single-celled animal which 

 is able to do all that a many-celled animal can do without any of that 

 many-celled animal's organs, is much more complex and remarkable 

 .than is the so-called higher form. 



And lastly, even those organisms highest in the scale of life begin 

 that life with a single cell which in turn grows by that cell dividing 

 and becoming two, these tw r o four, these four eight, and so on until the 

 complete adult is reached. 



With this summary in mind we may take up the study of the single- 

 celled organisms. 



