CHAPTER IV 

 CIRCULATION 



Necessity for Circulation. — Assimilation and respira- 

 tion are rendered effective through circulation. As new 

 protoplasm may be made in any part of the body, oxygen 

 and food products, the materials involved in its manu- 

 facture, must be transported throughout the body, and 

 the wastes that result from its disintegration must be 

 transported to a place from which they may be ejected. 



The System. — In one-celled animals the circulation 

 of the protoplasm within the cell suffices for the distri- 

 bution of substances through the body. 



In many-celled animals a special circulatory system is 

 necessary, for substances taken in at a definite point 

 must be transported to tissues which may be remotely 

 situated. An elaborate network of tubes penetrate all 

 the tissues. These tubes are filled with blood which is 

 in constant motion, and through the blood the transfer 

 of substances from place to place is effected. 



In Lower Animals. — In the lower animals the tubes 

 are few in number and their comparatively simple ar- 

 rangement is determined by the shape and size of the 

 animal and the arrangement of the various organs in its 

 body. In starfish, for example, a blood vessel encircles 

 the mouth and sends off a branch into each one of the 



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