80 



ANIMAL BIOLOGY 



Animal Kingdom. They live literally everywhere from hot springs 

 to Arctic ice, from desert sand to bottom mud of lakes and seas, 

 from the roots of plants to the blood of Man. A thimbleful of soil 

 may contain hundreds, even thousands, of Roundworms. Esti- 

 mates indicate that there are upward of twenty thousand species 

 parasitic on Vertebrates alone, and twice as many more if all the 

 other parasitic and free-living species were catalogued. 



The Roundworms are slender, cylindrical worms as is indicated 

 by other common names, such as Threadworms and Hairworms. 

 Their body plan shows, in general, considerable advance over 

 that of the Flatworms, because the simple intestine has two open- 

 ings to the exterior, mouth and anus; the nervous system consists 

 of a nerve ring about the pharynx from which arise large nerves, 

 extending the length of the body; the excretory system is well 



Uteri r 

 Excretory tube \ Ovary \ \\ /Pharynx 



Genital pore 



Excretory pore 

 Mouth 



Roundworm, Ascaris lumbricoides: diagram of dissection from 



right side. 



developed; and the male and female reproductive systems are 

 usually in separate individuals. Furthermore the various organs 

 lie in a spacious body cavity. However, there are no special cir- 

 culatory or respiratory systems. (Fig. 44.) 



Naturally the Roundworms that have been most studied are 

 parasites of Man and domestic animals. The Guinea worm is some- 

 times several feet long, and spends its adult life just under the 

 human skin and its youth in a Water-flea. The various species of 

 Ascaris are relatively large intestinal worms, the females attaining 

 a length of more than a foot and producing some fifteen thousand 

 eggs a day. Rut two of the most dreaded parasites are the tiny 

 Trichinella, the cause of the often fatal trichinosis contracted 

 by eating infected pork, and Necator, notorious as the Hookworm. 

 All considered, the Roundworms are hardly second to the Flat- 

 worms from the standpoint of medical zoology and, like them, 

 tax the ingenuity of biologists in ferreting out their complex re- 

 productive processes and life histories that have been evolved 



