WORMS. 191 



faces, but no general definition can be formulated for 

 them. 



2. The chief groups of Yermes are Plathelminthes, Ne- 

 mathelminthes, Annelida, and Molluscoida. 



3. The PLATHELMINTHES have no body-cavity and only 

 one opening (mouth) to the digestive tract, when this is 

 present. 



4. The Plathelminthes are divided into Turbellaria, 

 Trematoda, and Cestoda. 



5. The Trematoda are parasitic on or in other ani- 

 mals, often causing serious disease. They have a three- 

 branched alimentary canal, and adhere to the host by 

 suckers or hooks. 



6. Many trematodes pass through two hosts and present 

 also an alternation of generations in their life-history. 



7. The Cestoda are all internal parasites and have lost 

 the digestive system. 



8. Many become broken up into proglottids, new pro- 

 glottids forming in front while the older ones at the 

 hinder end drop off when mature. 



9. They frequently have an alternation of hosts. Man 

 gets his cestode parasites (tapeworms) from the cow and 



Pig- 



10. The NEMATHELMINTHES have cylindrical bodies, and 

 a complete alimentary canal (with mouth and anus) which 

 runs through a large undivided body-cavity. 



11. They show no segmentation of the body and repro- 

 duce solely by eggs. 



12. They are largely parasites. The Trichina is one of 

 the worst of human parasites, frequently causing death. 



13. The ANNELIDA have segmented bodies with external 

 ringing of the body and a division of the body-cavity into 

 paired pouches. 



