Summary of the Frog 121 



and the digestive process consisting largely of fermentation and the 

 absorption of such digested food, but also as including the taking in 

 of oxygen through the respiratory tract to cause heat and energy, and 

 the distribution through the circulatory system of the blood. And 

 finally, there must be included the excretory system which eliminates 

 all that for which the body has no further use. 



An organism was defined as any living thing capable of leading an 

 independent existence. 



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 consequently 

 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 which are 

 done by the different organs in the frog. 



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 a division of that 

 cell into two, then these two become four, these four eight, and so on 

 until complete adultship is reached. 



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

 celled organisms. 



