xii BACKBONELESS ANIMALS 235 



are internal boarders have an intricate life-cycle, requiring 

 to pass from one host to another of a different kind if 

 their development is to be fulfilled. Thus the liver- 

 fluke (Distomum hepaticum), which causes the disease of 

 liver-rot in sheep, and has sometimes destroyed a million 

 in one year in Britain alone, has an eventful history. 

 From the bile-ducts of the sheep the embryos pass by the 

 food-canal to the exterior. If they reach a pool of water 

 they develop for 2-3 weeks, quit their thick egg-shells, 

 and become for a few hours free-swimming. The active 

 larvae, known as miracidia, knock against many things, 

 to which they pay no heed, but when they come in 

 contact with a small water-snail (Limncea truncatula) 

 they fasten to it, bore their way in, and, losing their 

 locomotor cilia, encyst themselves. The encysted forms, 

 known as sporocysts, grow and multiply in a remarkable 

 asexual way. Cells within the body of the sporocyst 

 develop into a second generation quite different in form. 

 These are known as rediae, and each sporocyst forms 5-8 

 of them. Within the rediae more rediae (8-12) are produced, 

 and the last generation of rediae produce (12-20) minute 

 tailed flukes or cercariae. In winter there may be but 

 one generation of rediae. The cercariae leave the mori- 

 bund snail, leave the water, climb on to blades of grass 

 near the pool, lose their tail, and encyst. They look 

 like little white spots. If they happen to be eaten by 

 a sheep, they pass into the liver and develop in about 

 six weeks into fully- formed flukes. Others have not less 

 eventful life cycles, but that of the liver-fluke is most 

 thoroughly known. When we dissect a frog we often 

 find Polystomum integerrimum in the lungs or bladder ; 

 it begins as a parasite of the tadpole, and takes two or 

 three years to become mature in the frog. Quaint are 

 the little forms known as Diporpa which fasten on the 

 gills of minnows, and unite in pairs for life, forming 

 double animals (Diplozoon) ; and hardly less strange is 

 Gyrodactylus, another parasite on freshwater fishes, for 

 three generations are often found together, one within 

 the other. The most formidable fluke-parasite of man 

 is Bilharzia, common in Africa, which* like the Liver-fluke, 



