EFFECTS OF PARASITISM 201 



develop into free-living larvae which are swallowed by or burrow 

 into a new host when opportunity offers, e.g., the hookworms. 



Most of the intestinal parasites apparently enter their host 

 by way of the mouth, and the eggs escape with the faeces. Many 

 species enter as larvae in the tissues of an intermediate host which 

 is eaten by the final host. Of such a nature are most of the 

 tapeworms and flukes and some nematodes, e.g., Trichinella. 

 Some nematodes of the intestine, as the pinworm and whip- 

 worm, enter contaminated food or water as fully developed 

 embryos in the eggs. Still other species, as the hookworms and 

 Strongyloides, usually reach their destination in an indirect way 

 by burrowing through the skin. All the intestinal worms except 

 Trichinella produce eggs or larvae which escape from the body 

 with the faeces. In Trichinella the larvae encyst in the muscles 

 and in order for them to be released the host must be eaten by 

 another animal. Many of the worm parasites of other organs 

 of the body also enter by way of the mouth and digestive tract, 

 though they have various means of exit for the eggs or larvae. 

 The liver flukes enter and escape from the body as do ordinary 

 intestinal parasites; the lung flukes enter by the mouth, but the 

 eggs are expelled with sputum; the blood flukes enter by bur- 

 rowing through the skin, and the eggs escape either with faeces or 

 urine; the Filarice, like blood-dwelling protozoans, enter and 

 leave the body by the aid of blood-sucking insects; the guinea- 

 worm enters by the mouth, and the larvae leave through the skin. 

 The larval tapeworms which infest man enter either by the 

 mouth or by accidental invasion of the stomach from an adult 

 in the intestine. Like Trichinella they are usually permanently 

 sidetracked in man, since they can escape only by being eaten 

 with the tissues in which they are imbedded. 



Effects of Parasitism. — The effects produced by parasitic 

 worms depend in part on_the organs or tissues occupied, in part 

 on the habits of t he worms and in p art on the poisonous qualities 

 of their secretions or excretionsTt^o which the susceptibihty of 

 different individuals is very variable. The effects of some kinds 

 of worms is a much disputed point. Some investigators tend to 

 minimize the damage done by worm parasites, especially intestinal 

 ones, while others undoubtedly overestimate it. Improved facili- 

 ties for discovering infection have demonstrated the presence of 

 intestinal parasites in so many unsuspected cases that we are 



