198 INTRODUCTION TO THE WORMS 



on aquatic or land animals. They are flattened animals, usually 

 oval or leaf-shaped, furnished with suckers for adhering to their 

 hosts. The flukes which live as external parasites of aquatic 

 animals have a comparatively simple life history, while those 

 which are internal parasites of land animals have a complex 

 Ufe history, in the course of which they pass through two or three 

 different hosts. The third class, Cestoda, is comprised by the 

 tapeworms. As adults they are all parasites of the digestive tracts 

 of various animals and are profoundly modified for this kind 

 of an existence. Their peculiar method of multiplication by bud- 

 ding results in the formation of a chain of segments, sometimes 

 of great length, which collectively constitute a tapeworm; each 

 segment, however, is practically complete in itself and capable of 

 separate existence if it had some method of retaining its position 

 in the host's intestine. Some tapeworms have a life history 

 comparable in its complexity with that of the flukes but as a 

 rule it is much simpler. With the flatworms are usually asso- 

 ciated the Nemertinea, marine worms some of which are more 

 or less parasitic. None of them is of any interest in connection 

 with human parasitology. 



Roundworms. — Of somewhat higher organization than the 

 flatworms is the phylum Nemathelminthes or roundworms. 

 These worms are cylindrical instead of flattened, they possess a 

 body cavity, and they have an opening at each end of the digestive 

 tract (Fig. 60B). The excretory system usually consists of simple 

 tubes running the length of the body. The presence of a fluid- 

 filled body cavity through which food and other substances can 

 diffuse obviates the necessity for having branched organs. The 

 sexes are separate, and the reproductive systems are much 

 simpler than in the flatworms. 



Usually there is only a single class recognized as belonging to 

 this phylum, namely, the Nematoda or nematodes. Some of 

 the nematodes are not parasitic but many of them parasitize 

 either plants or animals. There are many important human 

 parasites among the Nematoda, for instance, the hookworms, 

 pinworms, Ascaris and other intestinal worms, Trichinella, Filaria 

 and the guinea-worm. In some of these the life history is fairly 

 simple while in others it is more complex and involves two dif- 

 ferent hosts. 



Some zoologists associate with this phylum two other classes 



