240 ZOOLOGY. 



on the right of Fig. 138. They may prove fatal, 

 owing to the pressure they exert in the organs con- 

 taining them and the blood-vessels of the same. 



The Expanded Tapeworm (Toenia expansa) 



lives in the intestine of the ox, where it may attain 

 the length of two feet, while in the intestine of the 

 Iamb it reaches at most the length of about nineteen 

 inches. It is sometimes present in large numbers in 

 lambs, causing difficulty of digestion with resulting 

 emaciation, poverty of blood, and even death. Bladder- 

 worm still unknown. 



Order : Trematoda (Flukes). 



Flattened parasitic worms, tongue-shaped or leaf- 

 shaped, living on or in a host, and provided with one . 

 or two ventral suckers. The intestine forks into two 

 branches immediately behind the gullet, and these 

 two divisions in some species {e.g. the large liver- 

 fluke) branch again repeatedly in a tree-like fashion. 

 Anus and blood-vessels absent. Almost all flukes 

 are hermaphrodite; they fertilize themselves. All 

 the forms living within a host (i.e. endoparasitic forms) 

 lay an immense number of small eggs, from which 

 larvae are hatched that become asexually reproducing 

 animals. These or their progeny produce once more 

 the sexual form. The development is therefore an 

 example of metagenesis (p. 16), but there is a change 

 of host. Only the two liver-flukes need be dealt 

 with here. 



The Large Liver Fluke {Distoma hepaticum), 

 (Fig. 141), is flat, but swollen in the middle, when it 

 contains a large number of eggs, under which circum- 

 stances the edges only are flat. The last are of a 

 dirty brown colour, while the middle of the body is 

 greyish yellow. There is a triangular projection at 

 the front end of the body with a sucker at the apex, 



