Chap. 25 FLATWORMS VANGUARD OF THE HIGHER ANIMALS 503 



Excretion and Water Balance. The excretory system consists of many large 

 flame cells each of which faces into a kidneylike (protonephridial) tubule 

 (Fig. 25.4). A network of these tubules opens out on the surface of the body 

 by minute pores. As in other animals, water is continually coming in and going 

 out of the body. Water that regularly diffuses into the body and collects in the 

 flame cells is waved into the tubules and passes through the microscopic out- 

 lets, thus completing the circuit. Nothing is known about the excretion of 

 nitrogenous waste. 



Nervous System. Planarians have a bilobed brain from which two main 

 nerves reach backward through the body giving off frequent branches (Figs. 

 25.3, 25.5). By skillful operating the brain can be removed. Following this 

 the animals remain quiet, unless stimulated, then they move about freely show- 

 ing that muscular action is independent of the brain and can be coordinated 

 by the branches. 



The sensory cells with which the head is richly supplied have already been 

 mentioned. Planarians are responsive to chemical substances, to changes of 



Excretory bulb 

 or flame cell 



Excretory 

 bulb 



Excretory 

 pore 



Excretory 

 canals 



Nucleus 



Cilia 



Cavity within the cell 

 opens into canal 





:) 



Excretory 

 canal 



Excretory 

 bulb 



Excretory 

 canals 



c. 



Excretory 

 opening 

 (pore) 



Fig. 25.4. The excretory system of a freshwater planarian. A, the entire system, 

 excretory bulbs (flame cells) in which excess water and metabolic waste is col- 

 lected and waved by cilia into the microscopic canals which finally carry it out 

 of the body through microscopic pores. Detail of canals: arrows mark the flow of 

 fluids from the bulbs. Highly magnified excretory bulb, called a flame cell from the 

 flicker of the cilia, which project into the cup-like cavity in the cell and create 

 a current of fluid into the canal. Like all kidney systems the function of this one 

 is the regulation of water content and the elimination of metabolic waste, espe- 

 cially nitrogen. 



