228 THE ECONOMIC IMPORTANCE OF ANIMALS 



of a round headlike part provided with hooks, by means of which 

 it fastens itself to the wall of the intestine. This head now buds 

 off a series of segmentlike structures, which are practically bags 

 full of sperms and eggs. These structures, called proglottids, 

 break off from time to time, thus allowing the developing eggs to 

 escape. The proglottids have no separate digestive systems, but 

 the whole body surface, bathed in digested food, absorbs it and is 

 thus enabled to grow rapidly. 



Roundworms. Still other wormlike creatures called round- 

 worms are of importance to man. Some, as the vinegar eel found 

 in vinegar, or the pinworms parasitic in the 

 lower intestine, particularly of children, do little 

 or no harm. The pork worm or trichina, how 

 ever, is a parasite which may cause serious 

 injury. It passes through the first part of its 

 existence as a parasite in a pig or other verte 

 brate (cat, rat, or rabbit), where it lies, covered 

 within a tiny sac or cyst, in the muscles of its 

 hosts. If raw pork containing these worms is 

 eaten by man, the cyst is dissolved off by the 

 action of the digestive fluids, and the living 

 trichina becomes free in the intestine of man. 

 Here it reproduces and the young bore their way 

 through the intestine walls and enter the muscles, 

 causing inflammation there. This causes a pain 

 ful and often fatal disease known as trichinosis. 



The Hookworm. The discovery by Dr. C. 

 W. Stiles of the Bureau of Animal Industry, 

 that the laziness and shiftlessness of the " poor whites " of the 

 South is partly due to a parasite called the hookworm, reads like 

 a fairy tale. 



The people, largely farmers, become infected with a larval stage 

 of the hookworm, which develops in moist earth. It enters the 

 body usually through the skin of the feet, for children and adults 

 alike, in certain localities where the disease is common, go bare 

 foot to a considerable extent. 



A complicated journey from the skin to the intestine now fol- 



Trichinella spiralis 

 imbedded in 

 human muscle. 

 (After Leuckart.) 



