398 ANIMAL BIOLOGY 



a new lease of life involving a series of changes. After about 

 two weeks it has become a sac-like creature, or sporocyst, which 

 in turn proceeds to develop within itself a brood of another larval 

 stage, the redia. Each redia liberated from a ruptured sporocyst 

 usually gives rise to one or more generations of redia, and the 

 final generation of these produces a third kind of larva, known as a 



CERCARIA. 



All these stages have been arising in the snail's body, but now 

 the swarm of cercariae emerges from the snail, and each swims 

 about in the water and finally encysts on a blade of grass. Here 

 again the life of the parasite hangs in the balance, for death fol- 

 lows unless the grass with the cyst is eaten by a sheep, and 

 the cyst reaches the animal's intestine. This location success- 

 fully attained, the cercaria escapes from the cyst, and makes 

 its way to the bile ducts where it soon develops into a mature 

 Liver Fluke, the cause of liver-rot in sheep. The life history is 

 completed. 



The large number of eggs produced by a single Fluke increases 

 the chances of a ciliated larva meeting the proper kind of snail, 

 while the various generations within the snail multiplies many- 

 fold the number of cercariae from a single egg, and just to that 

 extent increases the opportunities for at least one to reach another 

 sheep. This life history, while remarkable, is by no means unique, 

 and is presented as a type which is broadly representative of a 

 large group of parasitic Flatworms. No wonder that years of study 

 are required by specialists in different branches of zoology to 

 discover the various stages of the different species and determine 

 their relationships. 



Cestodes. The group of Flatworms known as the Cestoda 

 comprises the numerous species of Tapeworms which infest the 

 lower animals and Man. The best known species are Taenia 

 solium and Taenia saginata. both living as adults in the human 

 digestive tract, while the larvae of the former infest Pigs, and 

 those of the latter, Cattle. 



Taenia is a long ribbon-like worm comprising a small knob-like 

 head, or scolex, which is an organ for attachment to the lining 

 of the human digestive tract, and a large number of similar seg- 

 ments, or proglottides. These are formed by growth just be- 

 hind the scolex so that the oldest and largest proglottides are 

 at the posterior end of the animal. (Fig. 252.) 



