ROUNDWORM (aSCARIS) 337 



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 and the whipworm are well-known 

 examples. 



Class Gordiacea (gor di a she a, a knot). — Superficially these 

 animals resemble the nematodes, but the fundamental structure is 

 quite different, and therefore, it is likely proper to give them the rank 

 of a class. They are free-living as adults, but as larvae they are para- 

 sitic on larval May flies and other insects. They leave this host and 

 take up abode in a terrestrial form like that of grasshoppers or beetles. 

 After complete development the adult ''hair snakes" escape into 

 the water of some stream, puddle, or watering trough. These fe- 

 males again lay eggs in the water in long strings. In the adult 

 worm the intestine is a straight tube, often without a mouth, but 

 opening at the posterior end by an anus. Some have no intestine 

 at all. The outer surface of the body is covered by a cuticle. The 

 body is cylindrical and without lateral lines, excretory organs, or 

 circulatory system. Gor dins aquaticus and Paragordius varius are 

 the common examples of the group. 



Class Acanthocephala (a kan tho sef a la. thorn head) includes a 

 group, known as "spiny-headed worms," which is absolutely parasitic 

 in its habits. The adults are from a few millimeters to fifty milli- 

 meters in length and have an elongated, flattened body when found in 

 the intestine of a vertebrate but become distended to a cvlindrical 

 shape when removed to some solution outside the body. The protru- 

 sible proboscis is a peculiar and characteristic structure located at the 

 anterior end of the body. It bears numerous recurved hooks or 

 spines, and in many species it is capable of receding into a proboscis 

 receptacle or sheath: There is no^ digestive tract in this parasite, 

 and its food is absorbed through the surface of the body even though 

 it is covered with a cuticle. A single ganglionic mass constitutes 

 the central nervous system 



Habitat and Behavior of Ascaris 



Ascaris lumbricoides frequents the digestive tract of men and hogs. 

 It is entirely dependent on its host for furnishing suitable food and 

 environment. The only time this organism is at the mercy of the 



