260 BREEDING 



Insects, furnish not only most striking, but also easily recognized ex- 

 amples. 



Flukes, which occur in the ducts of the liver of various animals, horse, 

 sheep, and cattle, and tape-worms, which inhabit the intestines, are among 

 the most instructive instances of alternation of generations. 



A few lines will suffice to describe the curious metamorphoses, the 

 elucidation of which has occupied scientists for years of patient labour. 



Stock-owners are well aware of the effects of the invasion of the liver 

 of the sheep by the common fluke. This parasite is in form something like 

 a flounder or minute sole, about an inch in length when fully grown; its 

 digestive tubes are usually filled with bile. The reproductive system is 

 highly developed, male and female organs existing in the same creature. 

 Millions of eggs are deposited in the ducts of the liver of the sheep and 

 other animals, and carried into the intestines along with the bile, finally 

 being expelled along with the excreta. Falling on moist ground, the eggs 

 are hatched, and from them emerge not young flukes, but long, ciliated 

 embryos, as much unlike the parent as the most erratic imagination can 

 realize. Now the changes begin; the long embryo swimming about finds 

 a snail, the shell of which it pierces, and lodges itself in the body of the 

 animal, and becomes a " sporocyst", which means a cell full of germs. 

 This is the first generation. The germs are developed, and become more 

 highly organized than the embryos were, and are called " Rediae " (second 

 generation). The Redise escape from the parent cysts and lodge themselves 

 in various parts of the snail; meanwhile, inside these Redise long- tailed 

 Cercaria are developed (third generation). Some of the Cercaria, which are 

 tadpole-like creatures, wriggle out of the snail and enjoy for a brief space 

 a free life of swimming in the pools and puddles of wet grounds. Soon, 

 however, they fix themselves on grasses and other plants growing in water, 

 exude a gummy substance, and form little cysts, in which the Cercaria, 

 the inchoate fluke, is enclosed. In this state they remain until they are 

 swallowed by a sheep or other warm-blooded animal, when they escape 

 from their slight prison, find their way to the liver ducts, and assume 

 the form of minute flukes (fourth generation, from the egg of the parent 

 fluke). 



To put the case in one view, one fluke egg gives exit to one embryo, 

 which becomes one sporocyst, in which many Redise are developed. In 

 each Redia sac numerous Cercaria of the tadpole shape, the fluke of the next 

 generation, are formed and set free. Thus a single fluke egg is calculated 

 to be responsible for at least 200 Cercaria. Leuckhart has estimated that 

 the oviduct of a fluke may contain 45,000 eggs; it is only necessary to 

 multiply that number by 200 to arrive at the total number of young flukes 



