666 The Outline of Science 



likely to have pleasant lines. The mistletoe is pretty enough, 

 but as it is only a half-parasite it may be one of those excep- 

 tions that prove the rule. Our recoil is also in part due to a 

 recognition of the menace that many parasites involve. The 

 hookworm and the guinea-worm are curses. All the same there 

 is an astounding, undeniably romantic element in the life- 

 histories of many parasites. Let us take the liver-fluke as an 

 instance. 



The adult liver-fluke (Distomum hepaticum) lies like a flat 

 leaf in the tributaries of the bile-duct of the sheep and some other 

 mammals. It is about an inch long. It sucks in the blood of the 

 liver, and it causes the disease expressively called liver-rot. Like 

 most internal parasites, it is very prolific, each being capable of 

 producing 50,000 eggs, which are fertilised by sperms from the 

 same animal a rare state of affairs called autogamy. The de- 

 veloping enshelled eggs pass down the bile-duct, down the intes- 

 tine and on to the ground. If they land on a dry place, such as a 

 pathway, they soon die; if they come to rest in a damp place, e.g., 

 among wet grass, they continue developing for a time. But they 

 will not come to anything unless they reach a pool of water. Thus 

 drainage of pasture-land has greatly reduced the amount of 

 liver-rot. 



In the pool of water a microscopic, pear-shaped, ciliated larva 

 escapes from the egg-shell and swims about actively. It has two 

 minute eye-spots, but no mouth or means of feeding. So it cannot 

 continue swimming for more than a limited period, about eight 

 hours. 



During its swimming it comes near or in contact with many 

 things, such as stick and stone, water-weeds, and small animals, 

 but it answers back to nothing, save the touch of the little water- 

 snail (Lymnceus truncatulus) , which is very common in pools. 

 When the larval liver-fluke touches the mollusc it immediately 

 enters, finding the breathing aperture a convenient doorway. We 

 do not understand the specific irritability of the tiny brainless 



