THE THREE-LAYERED ANIMALS 



163 



diqcsf iv« +rac+ 



diqestivjc tract 



tonqHudinolnxiscles 



bcrve cord' cndoderrw 



circular muscles 



ec+oderro 



Fig. 9-3. A dorsal and sectioned view of planaria to demonstrate the systems, particularly the digestive. 



cell (Fig. 9-4). These cells have long cilia 

 which extend into the lumen of the canal 

 and it is their flickering motion that sug- 

 gests the name. The movement of the beat- 

 ing cilia carries nitrogenous wastes into the 

 lumen of the tube and to the exterior 

 throu2;h a number of excretory pores. Prob- 

 ably this system's chief function is in regu- 



lating the water balance, because most of 

 the nitrogenous wastes are lost through the 

 endodermal cells. Because the tubes of this 

 system are a primitive type of nephridium 

 (kidney), they are called protonephridia. 

 The nervous system of the planarian is 

 simple (Fig. 9-4), yet it is strikingly more 

 advanced than the primitive nerve net 



