THE WORMS 217 



and often is reduced to a mere bag through which the fluid food 

 prepared by its host is absorbed, Such animals as have lost 

 power to move about freely, or are otherwise changed by their 

 surroundings, are said to have degenerated. 



Sometimes a complicated life history has arisen from their para- 

 sitic habits. Such is seen in the life history of the liver fluke, a 

 flatworm which kills sheep, and in the tapeworm. 



Cestodes or Tapeworms. These parasites infest man and 

 many other vertebrate animals. The tapeworm (Tcenia soliwri) 

 passes through two stages in its life history, the first within a pig, 

 the second within the intestine of man. The eggs of the worm 

 are taken in with the pig's food. The worm develops within the 

 intestine of the pig, but soon makes its way into the muscles. If 

 man eats pork containing these worms, he may become a host for 

 the tapeworm. Another common tapeworm parasitic on man 

 lives part of its life as an embryo within the muscles of cattle. 

 The adult worm consists 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 struc- 

 tures, which are practically bags full of eggs. These structures, 

 called proglottids, break off from time to time, thus allowing the 

 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, par- 

 ticularly of children, do little or no harm. The pork worm or 

 trichina, however, 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 vertebrate (dog, cat, ox, or horse), where it encysts itself 

 in the muscles of its hosts. In the case of pork, if the meat is eaten 

 in an uncooked condition, 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 bores its way through the intestine walls 

 and enters the muscles, causing inflammation there. This causes 

 a painful and often fatal disease known as trichinosis. 



The Hookworm. The discovery by Dr. C. W. Stiles of the 



