CHAPTER XVIII 



THE ROUNDWORM, ASCARIS, OF PHYLUM 

 NEMATHELMINTHES 



This group is kno^vn as the unsegmented roundworms or thread- 

 worms. Some of the Nemathelminthes (nem a thel min' thez, thread- 

 worms) are free-living in soil, fresh water, and salt water; some are 

 found living in plant tissues; and others live in animal tissues as 

 parasites. The majority of them are microscopic, but a few are 

 macroscopic in size. These worms are long, slender animals whose 

 bodies are more or less cylindrical but tapering toward each end. 

 The range of length is from i/4 ^^' to four feet. They lack respira- 

 tory and circulatory systems, true coelom, and definite locomotor 

 organs. The group is very widely distributed and is deserving of 

 considerable attention. Some of the better known forms are Ascaris 

 (pigworm or eelworm), *' horsehair cnake," hookworm, pinworm, 

 Trichinella, Filaria, Guinea worm, whipworm, and eye worm. 



Classification 



Three classes are usually recognized in the phylum, although some 

 authors prefer to use only two. The three, classes are Nematoda, 

 Gordiacea, and Acanthocephala. 



Class Nematoda (nem a to' da, threadworm) is a group occupy- 

 ing almost every possible habitat capable of supporting life. There 

 are many free-living, fresh water, marine, and soil-inhabiting spe- 

 cies, and large numbers of parasitic forms living at the expense of 

 other animals and plants. This is a very important class parasiti- 

 cally. In size they range from Y^q mm. to more than a meter in 

 length. Locomotor organs are found in a few forms, no segmentation 

 is present, and there is no true coelom. 



Order Ascaroidea. — This group includes both parasitic and free- 

 living forms. Ascaris, the common intestinal worm, is the most 

 abundant. Enterohius vermicularis, the human pinworm; Strongy- 

 loides stercoralis, another parasite of man; Ascaridia lineata, the 

 chicken worm, and Toxocara canis of dogs are other familiar examples. 

 Ascaris lumhricoides will be discussed later as a typical example of 

 Nemathelminthes. 



335 



