430 The Animal Kingdom 



has been reported from over 800 different species of plants. In this 

 species, the infected plant forms very typical root knots about the 

 infested points. The sugar beet nematode, Heterodera schactii, has 

 been estimated to cause a loss of $129,000 per year on infested areas 

 of about 7,000 acres. The eelworm of vv^heat, Anguina tritici, has caused 

 a loss of as much as 80 per cent of a crop. Cobb found as many as 

 90,000 individual nematodes in a single gall produced by this species. 

 Certainly the importance of these forms cannot be overestimated. 



The Nematodes Parasitic in Animals. — All animals are sub- 

 ject to attack by nematodes. Except in rare instances, the nematode in- 

 festations do not result in death, for they are well-adjusted parasites. 

 More important is the fact that the animal is weakened and is liable 

 to attack by bacteria. Nevertheless fantastic numbers of individual 

 nematodes may be recovered from a single infestation. Approximately 

 5,000 individual pinworms were recovered from a man after but a 

 single treatment! Fortunately this number is an exception rather 

 than the rule. 



Man himself is subject to attack by at least 50 different species 

 of these worms. His domestic animals are equally populated : there 

 are 36 species that parasitize dogs, 33 for cats, 51 in cattle, 63 in 

 sheep, 69 in horses, and 33 in swine. In 1937 it was estimated that 

 there was an annual loss of domestic animals amounting to some 

 $500,000,000 in the United States from these important parasites. 



LIFE HISTORIES OF SOME IMPORTANT 

 NEMATODE PARASITES 



The life histories of the nematode parasites are as varied as 

 the species themselves. It is only through a knowledge of these life 

 histories that control measures can be effectively developed. 



Ascaris. — One of the commonest of the nematode parasites of man 

 is Ascaris lumbricoides (Fig. 141,^). This particular species is also 

 found in pigs, but there seem to be some important physiological 

 differences inasmuch as the human Ascaris will not infect the pig nor 

 can human beings in turn be infected by the pig Ascaris. 



The adult of Ascaris lumbricoides lives in the small intestine where 

 it feeds on the partially digested food of its host. There is some indica- 

 tion that the worms also may feed on the host's tissues. The female 

 worm is from 8 to 14 inches in length, may contain 27,000,000 eggs. 



