PH YL UM PL A THELMINTHES 



115 



or Mehlis' gland. The vitelline material which supphes both 

 the yolk cells and the substance of the shell is produced in 

 glandular bodies, the vitellaria, usually distributed along the 

 margins of the body (Fig. 64), and communicating with a 

 vitelline receptacle by means of smaller tubules which combine 

 to form two transverse ducts. 



Life Cycle. — As an illustration of the developmental cycle of 

 the Digenea, that of the sheep-liver fluke, Fasciola hepatica (Fig. 

 64), has been chosen. The adult flukes live in the bile ducts of 

 the sheep's liver where, by their 

 presence and by their munching 

 off portions of the lining of the 

 ducts, they produce a diseased con- 

 dition known as liver rot. As eggs 

 are discharged from the parent fluke, 

 they pass down the bile duct into 

 the intestine and out of the body of 

 the host along with the feces. The 

 embryo within the eggshell develops 

 into a small ciliated larva known as 

 a miracidium (Fig. 65 A) which is 

 released from the shell only when 

 surrounding conditions are proper. 

 In swampy ground or following a 

 heavy dew or rain, this miracidium 

 is enabled to swim in search of a 

 suitable snail before the trematode 

 can go further in its development. 

 Upon coming in contact with a snail 

 of suitable species, the miracidium 



makes use of a small boring spine and cercariae; £>, free cercaria; £;, 



on its anterior extremity to pene- y^/^^^,^ '^''^'^^- ^-^^"'^^^ "^'''' 

 trate the soft tissues of the snail and 



comes to lie in the liver of the snail. Here it undergoes a trans- 

 formation, becoming a bag-shaped structure called a sporocyst 

 (B). Parthenogenetic reproduction occurs within the sporocyst 

 to form numerous individuals called rediae. Each redia (C) is 

 a simply organized individual possessing a pharynx and tubular 

 intestine in addition to a single birth pore through which young 

 are discharged. Within each redia develops either a new genera- 

 tion of rediae or individuals of another type termed cercariae. 



Fig. 65. — Stages in the develop- 

 ment of the sheep-liver fluke 

 (Faficiola hepatica). A, miraci- 

 dium; B, sporocyst; with rediae 

 developing internally; C, redia, 

 with second generation of rediae 



