FLATWORMS, ROUNDWORMS, AND ROTIFERS 73 



Varied situations, and on water and land show this. Recall 

 that the rotifers lay two kinds of eggs to meet different 

 conditions and maintain an existence. 



Progression. — It is evident that the animals discussed 

 in these four branches are more highly developed than those 

 of the foregoing branch. Starting with the flatworms, we 

 find that many of them have a rather complete digestive 

 system and concentrated nervous system. The digestive 

 system of the flukeworm consists of a mouth, pharynx, gul- 

 let, and intestine, while the nervous system consists of a 

 ring of nerve matter around the gullet which gives off a 

 number of nerves, one pair running the length of the body. 

 Trichinella has a mouth, pharynx, and intestine, with a ring 

 of nerve matter about the pharynx that gives off nerves. 

 The rotifers possess a mouth, pharynx, gullet, stomach, and 

 intestine, — the most complete digestive system that we 

 have seen so far. They also have a small brain. 



CLASSIFICATION OF THE EXAMPLES 



Branch IV — Platyhelminthes. 

 Class — Treraatoda. 



Order — Digenetica. 

 Type of Order. 



Distomum hepaticum — Liver flukeworm. 

 Class — Cestoda. 



Order — Polyzoa. 

 Types of Order. 



Tcenia solium — Pork tapeworm. 

 Tcenia saginata — Beef tapeworm. 

 Branch V — Nemathelminthes. 

 Class — Neraatoda. 



Order — Nematoidsa. 

 Type of Order. 



Trichinella spiralis — Trichina. 



