54 



NATURAL HISTORY. 



The flukes and the tape-worms will represent the parasitic flat worms 

 in our pages. In various parts of the world sheep and other herbivorous 

 animals are occasionally victims of a disease known as 

 "sheep-rot" or "liver-rot," the latter term having refer- 

 ence to the degeneration of the liver, which results in death. 

 This disease is not so common in America as in Europe. 

 In the British kingdom the disease in 1879-80 destroyed 

 three million sheep. For a very long time it has been 

 W'$0M< known that a peculiar worm, the liver-fluke, was the cause 



ilftlPllI °^ tne disease, Dut tne °L uest i° n °f now it g ets f rom one 

 animal to another was not fully settled, although many 

 had worked at it, until 1882, when Mr. Thomas traced 

 the whole life history. A brief abstract of this is given, 

 because it is of itself interesting, and because it may be 

 used as a type of the subdivision of trematode worms. 



The scientific name of the fluke is Distoma (two mouths), 

 and alludes to the fact that the animal has two suckers 

 which were formerly supposed to be mouths. The adult 

 fluke occurs in the liver of sheep and other mammals, and 

 there produces its hundreds of thousands of eggs. These 

 pass out through the bile-ducts into the intestine, and 

 thence are cast out with the droppings over the field 

 OpMAoml where the sheep feed. Were another sheep to eat them 

 lum), enlarged. nQW ^ they would come to naught ; they need other stages 

 and only those eggs which may chance to find their way 

 into the pools of water in the pasture have any chance 

 of developing. These soon hatch out an embryo conical in 

 shape, and furnished with a retractile papilla on the larger 

 end. The whole outer surface is clothed with long cilia, by 

 means of which the embryo swims freely, or at times spins 

 on its longer axis like a top. This free life lasts about eight 

 hours, and if in that time the embryo has not found a host 

 in which to spend the next stage of existence, it dies. 



Many are the experiments that have been tried to ascer- 

 tain what this host may be, but so far only one form has 

 been found to act in this capacity, and this is a pond-snail 

 known to conchologi.sts as Lymncea truncatula. It is a small 

 species which is amphibious in its habits, being found as often on the land 

 as in the water. The embryo fluke seems to recognize at once the presence 

 of these snails among all the rest, and swim towards them. On touching 

 the body of the snail the motion of the embryo becomes changed to a 



Fig. 52. 

 water 



-A fresh- 



Fig. 53. — Young 

 embryo of the 

 liver-fluke before 

 it bores into the 

 pond-suail. 



