452 



ESSENTIALS OF ZOOLOGY 



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 



Fig. 192. — Life history of the liver fluke. Fasciola hepatica. A, adult in liver 

 of sheep ; B, freshly passed egg as it leaves the body of the host ; C, developing 

 embryo, ready to hatch in the water ; D, ciliated miracidium einbryo in the water 

 and about to enter the pulmonary chamber of snail E ; F, sporocyst containing 

 rediae ; G, redia containing daughter rediae ; H, redia of the second generation contain- 

 ing cercaria ; /, cercaria ; J, same having emerged from snail into water ; K, cercaria 

 encysted on blade of grass : L, cercaria liberated from cyst after ingestion by sheep ; 

 M, young fluke developing in liver of sheep. (Reprinted by permission frorn 

 Chandler, Introduction to Human Parasitology, John Wiley and Sons, Inc.;) 



