176 TEXTBOOK OF ZOOLOGY 



system is composed of a circumpharyngeal ring from which cords 

 extend posteriorly. It is a sensory-neuro-muscular system. The 

 structure of the free-living forms is generally more complex than 

 that of the parasitic forms. They are adapted to a wide variety of 

 habitats and can withstand many rigors of natural adversity, such 

 as freezing, high temperatures, droughts, and other unfavorable con- 

 ditions. Large numbers of free-living forms have not been named 

 and described. Representatives of this class have an intestine but 

 no proboscis. 



Order Ascaroidea. — It includes both parasitic and free-living 

 forms. Ascaris (Fig. 90), 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 lunibricoides will be discussed later as a typical example of 

 Nemathelminthes. 



Order 8trongyloidea. — This is an entirely parasitic group. The 

 males have caudal bursae with rays. The club-shaped esophagus is 

 without a posterior bulb. The hookworms of man, the Strongylus 

 roundworms of horses, and Syngamus trachea which causes gapes in 

 birds by obstructing the windpipe are all common representatives. 



Order Filaroidea. — This is a completely parasitic order, modified 

 for living in such tissues as lymph, blood, connective tissue, and 

 muscle of chorda te animals, and transmitted by certain insects. Two 

 distinctive characteristics are: (1) lack of bulb on esophagus; (2) 

 lateral paired lips or entire absence of lips. Guinea worm, eye worm, 

 and Filaria are the common humaai parasites. Some species cause 

 elephantiasis (Fig. 390) through occlusion of blood and lymph vessels. 

 This disease results in enormous swelling of the ai^ected parts. These 

 organisms are transmitted by mosquitoes. Several Filaroidea are 

 parasites of horses and dogs. 



Order Dioctophymoidea. — This is another parasitic group living 

 in the kidneys, body cavity, and alimentary canal of mammals and 

 birds. The genus Dioctophyme includes the largest roundworms, 

 some reaching more than three feet in length. 



Order Trichinelloidea. — This parasitic group has a peculiar cuticle 

 lining the esophagus, outside of which is a single layer of epithelial 

 cells. The common trichina (Fig. 396) and the whipworm are well- 

 known examples. 



