294 General Biology 



and dark-brown, with sharply denned 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 (pork tapeworm), (Fig. 183), 

 is the best laboratory example of Cestoda. It lives in the digestive tract 

 of man and feeds upon the already digested food of its host. The tape- 

 worm, 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 

 between 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 many species of tapeworms, but all live as parasites in 

 the intestinal tract of other animals, and nearly all require two hosts 

 before their life-cycle is completed. And, somewhat 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 

 then come in contact with grass, weeds, hay, or any vegetation which 

 cattle or hogs eat, they hatch in the intestine of the animal eating such 

 vegetation. In the case of the pork tapeworm, each Qgg will develop 



