202 ASSOCIATION OF ORGANISMS THE WEB OF LIFE 



producing eggs. The adult is therefore a compound animal, for 

 each stroke of the X was originally a distinct individual, and has 

 a mouth at one end. 



A great many Flukes live in the internal organs of various 

 animals, and differ in several important respects from those 

 already described. There is less occasion for adhesive organs, 

 and the usual number of suckers is two, one surrounding the 

 mouth at the front end of the body, and the other situated upon 

 the under surface. In some cases the lajter is absent. Two or 

 three different kinds of host are infested in the course of the life- 

 history, the reason probably being that unlimited increase in the 



same sort of host might ultimately lead to 

 its exinction, when a similar fate would 

 overtake the parasite. Although the re- 

 lations existing between the two or three 

 hosts are such as to favour transfer, the 

 chances are greatly against the survival of 

 a given larva, to meet which contingency 

 immense numbers of small eggs are pro- 

 duced, fewer and correspondingly larger 

 eggs being the rule for external parasites 

 attacking but one kind of host. And, as 

 might^ be anticipated, the life-history is very 

 complicated. 



The best-known form is the notorious Liver- Fluke (Fasciola 

 hepatica, see vol. i, p. 443), which, when adult, infests the liver of 

 the sheep, producing what is known as " liver rot". To this and 

 other especially injurious species reference will be made later. It 

 will suffice here to describe a form (Distomum macrostomum, fig. 

 1151) in which the life-history is rather simpler, but at the same 

 time of greater interest. This little Fluke, when adult, lives in 

 the intestine of various small birds, such as sparrows, warblers, 

 and tits. Its eggs pass out to the exterior, and many of them get 

 scattered over leaves. If one of them happens to be swallowed 

 by a particular species of small Snail (Succinea putris} it hatches 

 out into a minute larva, which bores through the wall of the 

 digestive tube, and penetrates between the organs contained in 

 the body of its host. Being now surrounded by nutritious and 

 easily-absorbed fluid it grows rapidly, becoming converted into 

 a shapeless sac (sporocyst), from which branches are given off in 



