CHAPTER 19 



THE TRANSPORTATION SYSTEM 



In the previous two chapters we have transportation because all of the cells are 

 seen that as animals grow larger, some sort in direct contact with the external environ- 

 of transportation system becomes impera- ment (Fig. 19-1). To some extent this is 

 tive to insure food and oxygen reaching the also true of planaria. However, its deep- 

 cells. Above the simple Metazoa such a lying cells, such as those in the mesoderm, 

 system has evolved in the form of a circu- may receive their oxygen and food from a 

 lating fluid in which the food and oxygen fluid, hemolymph, that bathes all of these 

 are transported. internal cells. There is no means of circu- 



Amoeba and hydra have no need for such lating the fluid except by the body move- 



490 



