XIV. CIRCULATION IN THE LOWER ANIMALS.* 



In the one-celled animals, such as the amcBba, the food 

 is digested in the cell protoplasm itself, and oxygen is 

 absorbed directly from the sm-rounding air or water. In 

 like manner the wastes and carbon dioxide are removed 

 from the cell and cast into the sm-rounding medium. In 

 such animals a circulatory system is evidently unneces- 

 sary. In fact, the only suggestion of it is found in the 

 movement of the protoplasm in the cell by means of which 

 the absorbed food is distributed throughout the cell. This, 

 however, is entirely an internal cell circulation, and not a 

 system for bringing food to the cells as in man. 



In the sponges and coelenterates each cell is able to 

 obtain its air supply and food supply directly, and no 

 special circulatory system is necessary. When we come 

 to animals possessing many layers of cells it is evident 

 that some cells will be so placed as to be imable to secure 

 food and air directly or to rid themselves of waste. Some 

 special apparatus, therefore, becomes necessary to supply 

 these cells with food and to remove their wastes. In 

 this way there arises the circulatory system as we have 

 considered it in our definition. Some of the systems in 

 the lower animals are interesting from the light they throw 

 upon the development of our own complex network of 

 tubes. 



Insect circulation. — In the grasshopper, we see one of 

 the simplest forms of a circulatory system. In this animal 

 ' See Footnote on p. 146, Chapter X. 

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