8o ECONOMIC ZOOLOGY AND ENTOMOLOGY 



the worms can take place such trichinosed flesh must be eaten 

 by another animal in which they can live. Pigs are probably 

 usually infected by eating dead infected rats or scraps of pork 

 that have been thrown out in garbage. Man is usually in- 

 fected by eating trichinosed pork. In the alimentary canal of 

 the new host the trichinae escape from the cyst and after be- 

 coming sexually mature produce the young which migrate to 

 the muscles again. 



The close watch that the government inspectors keep over 

 the meats that go out from all the great packing houses makes 



the dangers from all parasites of this 

 kind very much less than it used to be. 

 But the meat from the smaller slaughter 

 houses is usually not inspected and is 

 always a source of danger. No pork, 

 either fresh or smoked, should be used 

 without being thoroughly cooked in 

 order that any trichinae in it may be 

 killed. 



Eel-worms. Belonging to the genus 

 A scar is are several round- worms, or 

 "eel-worms," some of which are very 

 common and of considerable economic 

 importance. The large-headed thread- 

 worm, A scar is megalocephala, which occurs in the alimentary 

 canal of the horse, reaches a length of from eight to sixteen 

 inches and may be as thick as a lead pencil. The fertilized 

 eggs of the parasites are passed from the body of the host 

 with the excreta, and probably gain entrance to a new host 

 through stagnant water or by a horse eating grass or leaves on 

 which the eggs occur. 



A similar but somewhat smaller species, Ascaris lumbri- 

 coides, occurs in man and in sheep and hogs. In children, 

 particularly in tropical regions, these parasites may occur in 

 large numbers, sometimes from one to ten hundred in a 

 single individual. In such cases they may cause nervous- 

 ness, irritability, hysteria or even convulsions. In some cases 



FIG. 26. Encapsuled 

 trichinae in trunk muscle 

 of pig. (Greatly mag- 

 nified; after Braun.) 



