140 BIOLOGY 



Coordination. The numerous activities of the animal body 

 are brought into harmonious action for a common purpose. 

 The function by which they are related to one another is known 

 as coordination (Lat. con = together + ordinare = to regu- 

 late), and the system of organs that produces this coordination 

 is generally spoken of under the name of the nervous system. 



Reproduction. This is the function of producing new individ- 

 uals like the old, which prevents the species from disappearing 

 from the earth. 



The nine functions thus outlined are necessary to the life of 

 all animals. In a few of the lower animals, some of these func- 

 tions are very slightly developed; and in quite a number of 

 smaller animals we do not find any special system of organs 

 devoted to some of these functions. For example, many small 

 animals have no skeleton, and some of the very simple ones 

 have no organs that can properly be called a coordinating sys- 

 tem, since all of the functions of the animal take place in one 

 small cell where no coordination is needed. But speaking in 

 general, all animals, high or low, carry on all these functions. 



ANIMAL BIOLOGY 



In our consideration of animal Biology we shall study three 

 animals, chosen to illustrate different grades of structure. 

 Hydra will be an example of one of the simplest multicellular 

 animals; the earthworm, an animal of moderate complexity; 

 and the frog will be an example of the more highly complex 

 types. 



HYDRA FUSCA: A SIMPLE MULTICELLULAR ANIMAL 



General Description. The brown Hydra is a very common 

 water animal and may be found in almost any pond on the under 

 side of lily pads or pond weeds. Here it may be seen as a small 

 reddish body, just large enough to be visible. Our common 

 Hydra ( Hydra fusca) is of a brown color, but another common 

 species (Hydra viridis) is bright green. If the animal, still 



