234 



THE STUDY OF ANIMAL LIFE 



CHAP. 



1 worms,' 1 living outside or inside other animals, often 

 flat or leaf -like in form, provided with adhesive suckers. 

 Those which live as ectoparasites, e.g. on the skin of 

 fishes, have usually a simple history ; while those which 



FIG. 67. DIAGRAMMATIC REPRESENTATION OF THE LIFE-HISTORY OF THE 

 LIVER-FLUKE (Distomwn hepaticum). IT is SHOWN IN FOUR CHAPTERS, 

 TWO FREE AND Two PARASITIC. 



The adult fluke is shown at the top, about natural size. The anterior 

 terminal mouth leads into a suctorial pharynx (Ph), and this into the very 

 intricately branched food-canal (g) which has no anus. Behind the mouth 

 on the ventral surface is a muscular adhesive sucker (S). 



The next chapter to the right shows the free-swimming microscopic 

 larva or miracidium. It has only a few cells, the nuclei of which are shown. 

 It is covered with cilia and has two eye-spots (e). 



The third chapter is in the water-snail (Limntea truncatula). The 

 miracidium becomes a sporocyst, inside that rediae (R) are formed. Inside 

 the redia, which has a gut (g), more rediae develop, and there are in 

 summer two generations of rediae. In the last set cercariae ( C) are developed. 



The fourth chapter shows the cercaria, with a locomotor tail (<), an 

 attaching sucker (S), and the beginning of other organs. It leaves the 

 snail and the water, and encysts on grass a very minute spot. If it be 

 swallowed by a sheep, it develops into a fluke. 



