210 



THE FLUKES 



their bodies and these develop into a second generation of redise 

 (Fig. 63H) ultimately escaping from a httle " hatching pore " 

 in the body wall of the parent. In this way even a third genera- 

 tion of rediae may be developed, but usually the second generation 

 of redise produce from their germinative cells a new type of larva, 

 the cercaria (Fig. 631), quite different from either the embryo 

 or the redia. The cercarise are furnished with a sucker and a 

 forked digestive tract, and have an actively moving tail. They 

 worm their way out of the body of the snail in which they were 

 developed and swim about in the water by means of lashing 

 movements of their tails (Fig. 63 J). Eventually they attach 

 themselves to a submerged blade of grass or aquatic plant, lose 

 their tails, secrete a cyst about themselves (Fig. 63K), and wait to 

 be eaten or drunk by a sheep or a goat. When so swallowed the 

 cyst is dissolved off in the stomach, and the little parasite (Fig. 63L) 

 wends its way up the bile duct to the liver, there to begin again 

 the reproduction of eggs and start a repetition of the entire cycle. 



Such is the life history of the liver fluke. In some flukes this 

 strange life is further complicated by the invasion of a third host 

 by the cercarise. In some fluke parasites of frogs, for instance, 

 the redise inhabit certain snails, while the cercarise inhabit insect 

 larvae, and infect their ultimate host by being eaten with the 

 insects. Several human flukes, including the lung fluke, Para- 

 gonimus ringeri {westermani) have a life history of this type. 

 The encysted cercarise of the lung fluke are found in the tissues 

 of several species of fresh-water crabs and in the earlier stages 

 are believed to be parasites of a snail on which the crabs feed. 

 The Chinese liver fluke, Clonorchis sinensis, parasitizes succes- 

 sively a snail, a fish and a mammal. 



In some species of flukes daughter sporocysts are formed in- 

 stead of redise, and in some the sporocysts give rise to cercarise 

 directly. The known types of life histories of flukes are graphi- 

 cally shown by the following diagram, copied from Leiper: 



