ORGANIC EVOLUTION 



211 



the lungs on each circuit of the body. All the blood thus 

 gathers oxygen on each round. Hence, these are the 

 warm blooded animals: these alone are capable of sus- 

 tained activities in cold climates. 



The purpose of circulatory apparatus 

 is to get the food to the points where it 

 is needed for use, and to get the waste 

 to points whence it can be removed 

 from the body. When the animal body 

 is small and no part of it is far remov- 

 ed from food supply, there is little 

 need of circulatory apparatus. The 

 amoeba may feed at any point of its 

 body. In the hydra, the food cavity, 

 extending to the tips of the hollow ten- 

 tacles and out into the buds, is not far 

 removed from any cell. Even in so 

 large an animal as a flat worm the 

 food cavity may, by means of exten- 

 sive ramifications, reach nearly every 

 part. But in all the higher terrestrial 

 forms of animals, the part of the body 

 wherein food elaboration occurs is small, and the greater 

 part of the body is remote from food supply, and circula- 

 tion of the food is therefore necessary. Likewise, the 

 more the nephridia become localized in the body, the more 

 necessary becomes circulatory apparatus, with aefinite 

 blood channels leading to them from every part of the body. 

 But there is circulation before there are blood vessels. 

 We have seen it in Paramoecium; many of the lower 

 multicellular animals also lack blood vessels. There are 

 body fluids occupying the interstices between the cell layers 

 and bathing all the tissues internally. These are the media 

 through which the internal exchange of food and waste 



Fig. 135. Diagram of 

 double circulation 

 (from Verworn). 



