CBSTODES OK TAPEWORMS. 41 



sheep and oxen. These are said to be the same as the one 

 just described. These lung-fltikes are contained in cysts, usually 

 in the posterior lobe of the lung. The cysts are ovoid in form, 

 about the size of a walnut, and whitish-grey in colour. Ex- 

 ternally they are covered with a fibrous capsule. Each cyst 

 contains a fluid in which the fluke is found. 



The Polystomata are external parasites, and develop without 

 any alternation of generations such as we have just seen in the 

 liver-fluke of the sheep. Some species live in the bladder of 

 frogs {Polystomum interrimum), others live on the gills of fish 

 (P. lanceolatum). 



Cestodes or Tapeworms. 



The Tapeworms are a group of flat-worms which are entirely 

 parasitic in habits. They are segmented, but not in the same 

 sense as we understand the segmentation of the true Annelids. 

 There is neither a mouth nor an alimentary canal, as the nutri- 

 ment is taken in by the process of osmosis, absorption being 

 efi'ected through the skin of the whole body-surface. These 

 cestode worms, with a few notable exceptions (Bothrioeephcdus), 

 undergo a curious alternation of generations. The two stages 

 in the cestode life-cycle have often been described under two 

 different names. The two stages are — (1) the adult worm, 

 which is always found living in the intestines of man and 

 various animals and birds, &c. ; and (2) the " cystic stage," 

 which is usually found in a totally different host, and in some 

 of the organs or connective tissues of the body, not in the 

 intestines. The diseases produced by tapeworms are known 

 as tamiosis, and in some cases the disease ends fatally not 

 only in animals but also in man. A well-known instance of 

 this vermiceous group is the disease called " measles in pork," 

 which is produced by the cystic stage of a tapeworm (Tmnia 

 solium) that invades man. The worm is contracted by eating 

 infested pork insufficiently cooked. 



Tapeworms are found in all parts of the world, and are 



