. Animal biology. Zoology; Biology. 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
. Animal biology. Zoology; Biology. 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 hear
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