ANIMAL PARASITES 199 



starved the dodder may form chlorophyll and manu- 

 facture some food, but under favorable conditions it is 

 wholly parasitic. Other examples of parasitism among 

 the lower plants are described in later chapters in con- 

 nection with the subject of disease. 



Animal Parasites. — Parasitic species have been 

 found to occur in every one of the larger animal groups 



Fia. 48. — Mistletoe parasitic on oak. 



with the exception of the sponges and that to which the 

 starfishes, sea urchins and their relatives belong. In 

 some of these groups the list of such animals is com- 

 paratively small, while in others a large number have 

 assumed this mode of existence. 



Among the unicellular species, there are thousands that 

 are parasites, and, although the great majority infest the 

 lower animals, there are several that produce very seri- 

 ous human disorders. Various diseases of the digestive 

 tract, relapsing fever, sleeping sickness, and malaria, are 

 caused by one-celled parasites. Among the flatworms 

 also there are several hundred parasitic species famil- 

 iarly known as tapeworms (Fig. 50) and flukes. The 



