Phyla Plafyhelminthes and Nemertina 409 



Other Turbellarians. — Besides these representatives of the com- 

 mon genus Diigesia, other species occur in both fresh and salt waters. 

 Some of these Hve as ectoparasites or commensals, such as Bdelloura 

 which is frequently seen crawling over the booklungs of horseshoe crabs. 

 In the tropical forests, land planarians are commonly seen leaving their 

 slime trails over the damp, decaying vegetation. Some of these are 

 quite large and many are variously colored. 



One order of this class, the Acoela, has small forms ranging from 

 one to several millimeters in length. These show many primitive 

 characters which are important in tracing the possible evolution of the 

 flatworms. As the name implies, they lack an intestine. The ventral 

 mouth either leads directly into or through a short pharynx to a central 

 mass of digestive cells. These forms also lack an excretory system. 



THE CLASS TREMATODA 



The members of the class Trematoda are known as the flukes. 

 They are all parasitic, with the adults mostly occurring in vertebrates. 

 Many of these flukes are of great economic importance as parasites of 

 man and his domestic animals. There are two principal orders : the 

 Monogenea and the Digenea. 



The Monogenea, commonly called the monogenetic trcmatodes, are 

 ectoparasites, mostly living on the gills or skin of fish. Their life cycles 

 are simple, with the eggs hatching directly into young worms which 

 attach to the appropriate host and grow into adults. Sometimes in fish 

 hatcheries, these worms get so abundant as to destroy great numbers of 

 young fish. 



The second group, the Digenea or digenetic trematodes, have com- 

 plicated life cycles involving several changes of host. The animal itself 

 has a different form for each host, and often for the stages in between. 

 These are the most abundant and widespread of the flukes and contain 

 most of the economically important forms. 



The Life History of the Sheep Liver Fluke. — The complexity 

 of the life cycles of the digenetic trematodes is illustrated by that of the 

 sheep liver fluke, Fasciola hepatic a. The disease caused by this fluke, 

 liver rot, is most commonly found among sheep that are pastured in 

 moist meadows, and is found in many sections of the United States and 

 Europe. At times the infestation may become so heavy as to become 

 fatal to the host. 



