26 



Roundworms — Tlie Tubular Pi 



an 



Phylum Nemathelminthes — Nematodes 



Roundworms are spread over the earth in every region where animals Hve 

 (Fig. 26.1). Great numbers of them contribute to plant, animal, and human 

 welfare. Hosts of them live in the soil — minute, hidden, and little known. 

 Still others are parasites of plants, of invertebrate animals, and probably of all 

 vertebrates. 



Their evolution has included structures of very great importance to higher 

 animals. The tube-within-a-tube plan of the body first came into existence in 

 them, the digestive canal as the inner tube, the body wall as the outer one. 

 Less obvious in a peacock or a man, the plan is as really present in them as it 

 is in a hookworm or a vinegar eel, both of them roundworms. 



There are widely varying degrees of similarity and relationship among 

 roundworms. Formerly all of them were included in the Phylum Nemathel- 

 minthes. Now the more closely related roundworms are grouped together in a 

 phylum, the Nemathelminthes, by some zoologists and in a class, the Nema- 

 toda, by others. Still other more diverse forms are included in the small phyla 

 and the classes that are discussed briefly in the next chapter. 



Characteristics and Structure Illustrated by Ascaris. Nematodes are slender 

 worms, pointed at head and tail ends, many of them microscopic, others sev- 

 eral inches long. The structure of the widely distributed species of Ascaris 

 that parasitize man and pig is typical of nematodes rn general (Fig. 26.2). 



A Human Parasite. Ascaris lumhricoides is among the longest-known 

 human parasites and is still common in localities where the soil is polluted 

 with sewage. They probably became established in the human body when wild 

 pigs were first hunted and eaten, when agriculture was in its beginnings and 

 pigs were being domesticated. The human parasite {A. lumhricoides) is indis- 

 tinguishable except in habit from the Ascaris of the pig (A. lumhricoides, 

 variety suum) from which it doubtless originated. Probably infection with 



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