FLAT WORMS AXD THREADWORMS 293 



Opisthorchis (Distoma) sinensis. This is one of the most important 

 of liver-flukes. It occurs extensively in Japan, China and India. It is 

 10-20 mm. long and 2-5 mm. broad. The eggs are oval and dark-brown 

 with sharply defined operculum. O. sinensis are also found in Canada 

 and the United States. Children are usually affected, and whole villages 

 succumb to its ravages. 



Fasciolopsis (Distoma) buski is common in India, and 



Mesogonimus heterophyes in Egypt and Japan. 



CESTODA 



The common tapeworm, Taenia solium (Fig. 183) is the best labora- 

 tory example of Cestoda. It lives in the digestive tract of man and feeds 

 upon the already digested food of its host. The tapeworm therefore 

 needs no digestive system of its own, and it has none. 



Taenia is a long flatworm consisting of a knob-like head called the 

 scolex, and a great number of segments which are all like each other but 

 different from the scolex. These segments are known as proglottids. 



Hooks and suckers on the scolex permit the animal to fasten itself 

 to the walls of the digestive tract of its host. A small constriction be- 

 tween head and proglottids is called the neck. The proglottids usually 

 increase in size the further they are from the scolex. It is not uncom- 

 mon to have a tapeworm reach ten or more feet in length and have some 

 eight or nine hundred proglottids. The proglottids are budded off from 

 the neck, so that the segments furthest from the head are the older. The 

 process of forming new proglottids is called strobilization. 



The body of the simplest type of tapeworm is not segmented, though 

 most forms are. 



Each proglottid contains a set of both male and female reproductive 

 organs, but the nervous and excretory systems are usually quite con- 

 tinuous through head and proglottids. The question often arises as to 

 whether each segment is not a complete individual, but the best authori- 

 ties believe that the scolex is an asexual individual which buds off the 

 sexual individuals which we have called proglottids. 



There are a good many species of tapeworms, but all of them live 

 as parasites in the intestinal tract of other animals, and nearly all of 

 them require two hosts before their life cycle is completed. And, just as 

 the liver flukes require a cold-blooded and a warm-blooded animal as 

 their hosts, so the tapeworms usually require some herbivorous animal 

 as a host for the larval stages, and an animal which eats the flesh of the 

 herbivorous animal for the adult stages. We therefore have tapeworms 

 using pig and man, cow and man, fish and man, mealworm and rat, fleas 

 and dog, rabbit and wolf, etc., as the two hosts. 



An adult tapeworm in the intestine of man will continually develop 

 new proglottids which pass out of the body and shed the eggs upon the 

 ground. Each proglottid may produce thousands of eggs. If these eggs 



