36 



BRANCH PLATYHELMINTHES 



The trematodes are hermaphroditic, and the development may 

 be either with or without a metamorphosis. 



The Liver-fluke (Fig. 23) is parasitic in sheep. The eggs pass down the 

 bile-ducls of the sheep into the intestine, and from there to the exterior, 

 when the embryo escapes by the separating of the lid, or operculum, from 

 the egg-shell. 



The ciliated larva swims about in the water 

 or remains in the damp vegetation until it 

 comes in contact with a pond or land snail 

 (Fig. 24). It then bores into the body of the 

 snail, where it develops into a sporocysi 

 (Fig. 25), which produces rctlice. These rediw 

 possess a mouth, a pharynx, an intestine, and 

 an opening for the escupc of the young, which 

 are internally produced. According to the 

 season, these young are cennriae or redid, 

 several generations of which may follow 

 before the cercariw appear. The cercariw are 

 adapted for aquatic life. 



Fig. 26. — Free-swimming ccrcaria 

 of the common liver-fluke, greatly 

 enlarged. (After Leuckart.) 



Fig. 25. — Sporocyst of the com- 

 mon liver-fluke from the body of a 

 snail, containing redia; in course of 

 development — enlarged 200 times. 

 (After Leuckart.) 



The cercariw (Fig. 26) escape from the snail, swim about with their vibra- 

 tile tails for a time, when the tails drop off and the ccrcaria' become encysted 

 on a plant. When' this plant is eaten by a sheep, cow, or hog, the young 

 escapes from the cyst and makes its way up the bile-ducts to the liver, 

 where it develops into the mature worm and produces reproductive organs, 

 thus completing the life-cycle. 



Sheep pastured in swampy places are likely to be infected by this para- 

 site, and wet seasons cause epidemics. 



