762 



TEXTBOOK OF ZOOLOGY 



and crayfishes; Gotylophoron cotylophorum, a stomach parasite of 

 cattle in Louisiana, and Dicrocoelium lanccahnn, a common liver fluke 

 of herbivorous mammals in Europe and Asia. 



Numerous ectoparasitic trematodes, of the Subclass Monogenea, 

 occur on the skin and gills of fish and are often of economic im- 

 portance because they kill goldfish and other aquarium fishes, and 

 also young fishes in state fish hatcheries. 







\ 



Fig-. 401. — Development of tapeworm. A, six-hooked embryo ready to become 

 embedded in muscle ; B, cysticercus, or bladder worm as encysted ; C. section 

 through developing scolex in cysticercus; D, later stage; E, scolex everting as it 

 protrudes from bladder; F, extension of scolex from bladder; G, later stage ;^, 

 formation of proglottids. (From Parker and Haswell, Zoology, published by The 

 Macmillan Company, after Jijima and Hatschek.) 



The Tapeworms (Class Cestoda).— Cestoda differ from Trematoda 

 in the complete lack of a digestive system. In fact, Cestodes never 

 have any sign of a digestive organ at any time during life; they 

 receive their nourishment by absorbing through the surface of their 

 bodies the food already digested for them by the host. Most cestodes 

 also differ from trematodes by having the body divided into a series 

 of segments, one behind the other, each segment having a com- 

 plete set of reproductive organs. This structure characteristic of 



