178 



METAZOAN PHYLA 



A/fouth 

 Pharynx 

 Fsophagrus 

 A/eri^e ce//s 



A/e;rve 





A'/<?^/y^^ 



place through the anal opening and elimination is effected by the excre- 

 tory system. 



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

 lization is internal. After fertihzation 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. In case the eggs 

 are laid, they are provided with yolk and shells and when passed out of 

 the body contain embryos. The shell is often heavy and very resistant 



to chemicals which would injure 

 the organism. 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. 



210. Life History of the Pig 

 Ascaris. — The parasitic nematodes 

 possess some very interesting and 

 remarkable life histories. One of 

 these is that of an 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 it was 

 young. The eggs of the parasite, 

 after being passed out with the 

 feces and mixed with the soil in 

 the hog lot, are taken up by the 



Fig. 85. — Monhystera sentiens Cobb, a-^ feP& *-' j ^ 



free-living nematode. Side view of female. tO rOOt about; Or if infested SOil is 

 Probably the most widely spread nematode ^^^j^g^ ^ ^j^^ ^^^^ ^f ^^g 



genus, found m fresh water, m the sea, and m ^ 



soil. (From Cobb, in Ward and Whipple's mother, the eggs may be taken 



"Fresh-water Biology," by the courtesy of John ^^ ^Yien SUCkling. The eggS paSS 

 Wiley & Sons, Inc.) X 94. c i 



into the intestme oi 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 follow- 

 ing the free surfaces of the air passages to the pharynx, reach the alimen- 

 tary canal. Following this canal they return to the intestine to complete 

 their growth, mature, and reproduce. 



/^ 



Sae/-r» 

 ce//s 



^^^ c^//s 



