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 latter 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 (Fasczola 
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 (Destomum macrostomum, fig. 
1151) in which the life-history is rather simpler, but at the same 
time of greater interest. This littlke 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 (Swccznea 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 
Fig. 1150. —Diplozoén paradoxum 
