262 GENERAL ZOOLOGY 



or plants. Above there is a pair of small eyes of very simple 

 structure. The body is not divided into somites. Each indi- 

 vidual is hermaphroditic, but in many instances reproduc- 

 tion takes place asexually. In asexual reproduction the body 

 becomes constricted and separates into two parts, each of 

 which then grows the missing portion. Thus by fission two 

 complete individuals result from the division of a single 

 specimen. In addition to this normal means of asexual 

 reproduction planarians have extreme powers of recovery 

 from injury. Even a small fragment cut from the body of 

 one of these worms has the ability to produce a complete 

 new individual through the powers known as regeneration. 

 The class to which the planarians belong is called Turbel- 

 la'ria (Lat. turbo, a "whirling," referring to the movement of 

 water about the mouth). In all members of this class the 

 body is covered with minute hair-like structures called cilia. 



Parasitic Flatworms 



Though planarians may on occasion live as parasites, 

 there are two other classes of flatworms, the flukes, or trema- 

 todes, and the tapeworms, or cestodes, which are unable to 

 live in any manner other than as parasites. It is not un- 

 common for these parasitic flatworms to produce disease in 

 their hosts. Especially when they parasitize man or his 

 domestic animals they may be of very great economic 

 importance. 



Trematodes. There are many kinds of trematodes, rang- 

 ing in size from microscopic creatures barely visible to 

 the naked eye to large, flat, leaf-like worms more than an 

 inch long. Since they commonly live within other animals, 

 they are but rarely seen except by students of zoology 

 and medical men. 



The Liver Fluke. In North America one of the most im- 

 portant trematodes from the economic point of view has 



