88 NEMATHELMINTHES 



stitute one of the important mechanical as well as biological factors in soil 

 and in the bottom of lakes and oceans. Estimates based on (Dr. Cobb's) 

 investigations show that in the upper foot of an arable soil the numbers 

 of nematodes run to thousands of millions per acre." 



Longevity. — When in encysted condition in grains or in the soil, 

 nemas may live for years. Needham found in 1743 that nematodes 

 in wheat ears would live for several months in a dried condition. 

 Baker found that nematodes dried for twenty-eight years became 

 active again when moistened. (Becquerel. Latent Life. Scientific 

 Am. Supp., vol. 82.) 



Family i. Ascaridae. — In the nemas of this family the body is 

 thick, and the mouth has tnree lips always bearing papillae and 

 amphids. The males are smaller than the females and have a curved 

 caudal end. Numerous species attack the vertebrates and many 

 of the invertebrates, living as parasites in their intestines, but found 

 in other organs or in the body cavity. In general they require no 

 intermediate host. 



Type of the Group — Ascaris lumbricoides. — Jscaris lumbri- 

 coides, the human " eelworm," is found in the human small intestine 

 where resultant lesions may induce the symptoms of anemia.^ 

 Profound respiratory affections such as pneumonia may be caused 

 by ascarids lodged in the lungs. It has been estimated that from 

 10 per cent to 40 per cent of Europeans are infested with A. lum- 

 bricoides. 



The females contain as many as sixty million eggs. After 

 fertilization the eggs pass out of the body of the host with the 

 feces. The eggs become embryonated in the soil. Such ova remain 

 viable for five or six years. They enter the digestive tract through 

 water or contaminated food. Dirt eaters sometimes take them in 

 and it is possible that they may also enter with unwashed vegetables. 



Ascaris lufnbricoides has been found in the dog, sheep and hog. 

 It possibly occurs in the cat and the rat. The host relationship 

 of pig and human ascarids has been tested by feeding eggs of the 

 human ascaris to pigs. They induced respiratory disturbances but 

 did not become established In the digestive tract of any of the pigs. 



^ Wells, Jour. Paras., vol. 17, pp. 167-182, June, I931, found that a single dog- 

 hookworm, a strongyle, see p. 92, may withdraw .8 cc. of blood from the host in 24 

 hours. The Ascaridae may be extremely injurious also. The number of nemas as yet 

 known and studied is relatively so small that their classification is still provisional. 



