THE WEB OF LIFE 307 



guarded by its bearer as if it were a bunch of eggs. But 

 we cannot do more than give a glimpse of this wonder of 

 parasitism. 



In Illustration 



Liver -Fluke. — ^The well-known hfe-history of the hver- 

 fluke {Distomum hepaticum) affords vivid illustration of the 

 vicissitudes that are so common — especially, it may be 

 noticed, in the case of parasitic animals. The adult lies, 

 like a flat leaf, in the tributaries of the bile-duct of the 

 sheep (and some other mammals), causing the disease 

 known as hver-rot, which often does much damage among 

 sheep. Like most internal parasites, it is very prohfic, 

 and it is pecuhar inasmuch as it fertihzes its own eggs. 

 The developing eggs pass down the bile-duct, down the 

 intestine, and on to the ground. If they are deposited on 

 quite dry ground, they soon die ; if they come to rest on 

 damp soil or among wet grass, they may remain in a state 

 of latent hfe for a couple of weeks ; if they fall into a 

 pool of water, they continue developing. In a short time 

 there emerges out of the egg-envelope a microscopic, some- 

 what pear-shaped, ciliated larva, which swims freely in the 

 water. It has energy to continue swimming for about 

 eight hours, but has no mouth or means of feeding. In 

 the course of its swimming it comes into contact with 

 many things, such as stick and stone, water-weeds and 

 small animals, but it pays no heed to any until it happens 

 to touch the little water-snail {Lymnwus truncatulus or 

 minutus) into which it immediately enters, finding the 

 breathing aperture a convenient door. If we could 

 imderstand the memory of the hving matter which enables 

 this tiny brainless larva to respond effectively to the touch 



