PLATYHELMINTHES, 



Body elongated, flattened and unsegmented. Anus gener- 

 ally absent. 



Class 1. Turbellaria. 



Outer surface ciliated. Free living. 



Order 1. Polycladida. 



Intestine complexly branched. No separate 

 vitellaria. (Planocera, Leptoplana, Stylochus.) 



Order 2. Tricladida. 



Intestine with anterior median, and two posterior 

 lateral limbs. Vitellaria numerous. (Planaria, 

 Bdelloura, Syncoelidium.) 



Order 3. Rhabdocoelida. 



Simple, sac-like intestine. Body usually elon- 

 gated. (Polychcerus, Microstomum.) 

 Class 2. Trematoda. 



Parasitic. Generally with sucking disks. Well- 

 developed digestive system. 



Order 1. Monogenetica. 



Ectoparasitic. Direct development. Three or 

 more suckers. (Polystomum.) 



Order 2. Digenetica. 



Endoparasitic. Complicated development. Never 

 more than two suckers. (Distomum.) 

 Class 3. Cestoda. 



Endoparasitic. Without digestive cavity. Usu- 

 ally having a scolex, bearing clinging organs 

 (suckers or hooks). 



Order 1. Monozoa. 



Body not divided into proglottids. (Caryophyl- 

 laeus.) 



Order 2. Polyzoa. 



Body consisting of scolex and proglottids. (Tae- 

 nia, Crossobothrium.) 

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