PHYLUM NEMATHELMINTHES 



181 



A/foufh 

 Pharynx 



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absorption through the thin wall of the intestine into the body cavity. 

 By means of this cavity it is distributed throughout the body. Within 

 tissues it is passed from cell to cell. Egestion takes place through the 

 anal opening and elimination is effected by the excretory system. 



211. Reproduction. — The nemathelminths are all diecious, and ferti- 

 lization is internal. After fertilization those eggs which develop within 

 the body, as in the species of Trichinella, are lodged in the uterus, where 

 they hatch, the female bringing forth living young. The shell is often 

 heavy and very resistant to chemi- 

 cals which would inj ure the organ- 

 ism. The larva, which at first may 

 be free and may remain so in free- 

 living nematodes, enters, in the 

 case of all parasitic forms, into 

 another animal which may be 

 either an intermediate or a final 

 host, or into some plant. 



212. Life History of the Pig 

 Ascaris. — The parasitic nematodes S^^^"^ 

 possess some very interesting and 

 remarkable life histories. One of 

 these is that of the ascaris found 

 in the pig. The adult pigs are 

 immune from infection by this 

 parasite, which, if it is found in a 

 mature pig, must have entered it 

 when the host was young. The 

 eggs of the parasite, after being 

 passed out with the feces and 



mixed with the soil in the hog ^j^ ^i.—UonhysUra sentiens Cobb, a 



lot, are taken up by the young pigs free-living nematode. Side view of female. 



, , 1 • , 4. u i. Probably the most widely spread nematode 



as soon as they begm to root about; g^^^^^^ ^^^^^ -^ ^^.^^^^ ^ater, in the sea, and in 



or if infested soil is caked upon soil. (From Cobb, in Ward and Whipple's 

 ,, , 1 ~ ., ,, ,, "Fresh-water Biology," by the courtesy of John 



the body of the mother, the eggs j^^^^y ^ sons. inc.) x 94. 

 may be taken in when suckling. 



The eggs pass into the intestine of the young pig and by the destruction 

 of the shells the larvae are freed. The larvae then leave the intestine 

 by puncturing the wall and pass by way of the portal system of blood 

 vessels to the heart. From the heart they follow the pulmonary artery 

 to the lungs, make their way through the walls of the lung cavities into 

 the air spaces in the lungs, and then, by following the free surfaces 

 of the air passages to the pharynx, reach the alimentary canal. Follow- 

 ing this canal they return to the intestine to complete their growth, 

 mature, and reproduce. 



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