DISTINCTION BETWEEN ANIMALS AND PLANTS. 15 



colouring matter called chlorophyll (leaf-green), which 

 is characteristic of nearly all plants except Fungi and 

 parasites. This is disti-ibuted in granules. The few- 

 exceptions to this rule, that chlorophyll is characteristic 

 of plants, include two freshwater animals, common in 

 brooks, Hydra viridis, the freshwater polyp (see p. 160), 

 and SpongillcLf the freshwater sponge. Another dis- 

 tinction is the presence of a peculiar coating to the- 

 structural units of plants, which is called the cell-wall, 

 and consists of a substance called cellulose (see p. 16). 

 The existence of this can be demonstrated by reagents 

 which colour it. We will now pass on to describe 

 something of the microscopic structure of animals, for 

 without a clear understanding of this, the structures 

 which can be seen by the eye would have but little 

 meaning for the observer. Nothing will be said here 

 about the use of the microscope ; but it will easily be 

 understood that familiarity with its practical use is 

 absolutely essential for the student of zoology. The 

 reader who is not familiar with microscope work will 

 lind some useful information in Part III., chap. II. 



