246 



PLATHELMINTHES 



they possess either the oral sucker alone (Monostomum) or this and a 

 second ventral sucker, and only rarely other attaching apparatus. They 

 are markedly separated from the Polystomes by their life history. The 

 alternation of hosts necessitated by the endoparasitic life is complicated 

 by an alternation of generations (heterogony, p. 132) with metamorphosis, 

 hence 'Digenea.' To illustrate this the history of Distomum hepatic-urn, 

 the liver fluke of the sheep is chosen (fig. 220). 



The eggs leave the maternal uterus at the beginning of embryonic 

 development, pass down the bile ducts and thence by the intestine to the 

 exterior. They must come into water and remain here a while before 

 the ciliated larva (miracidium, A) escapes by lifting of the lid of the shell. 

 This larva bores its way into a small snail (sp. of Limncca), where it 

 grows into a sporocyst (B}. The sporocyst, a muscular sac with proto- 

 nephridia but lacking all other organs, produces in its interior eggs which 

 develop into a second reproductive sac, the redia (D). These are dis- 

 tinguished from the sporocysts by the possession of pharynx and a tubular 



intestine as well as a birth-opening for the 

 escape of the young produced inside. Ac- 

 cording to the season these young are either 

 cer carlo; (F), or another generation of rediae 

 may follow before the cercariae appear. 

 The cercariae are adapted for an aquatic 

 life, since each has, besides the characteristic 

 organs of a Distomum (genitalia excepted), 

 a strongly vibratile tail. The cercariae 

 escape from the snail, swim about in the 

 water until the tail drops off, when they 

 encyst on water plants. When these en- 

 cysted young are eaten by sheep along with 

 the vegetation, infection follows. 



In general it can only be said of the life 

 history of other Trematoda that the miracidia 

 must penetrate a mollusc, and that the different 

 species have many modifications. Best known 

 are the following: Distomum (Fasciolaria) 

 hcpalicum, the liver fluke (fig. 217), about the 

 size and shape of a pumpkin-seed, lives in the 

 bile-ducts of sheep, cows, pigs, etc., and rarely 

 of man. It causes a disease known as 'liver rot,' generally resulting in death. 

 This history shows why shep pastured in moist places are subject to the 

 disease, and why wet seasons are times of epidemics. Thus in the rainy year 

 of 1830 about one and a half millions of sheep were killed in England; in 1812, 

 300,000 in the neighborhood of Aries, France. It is frequently accompanied 

 by D. lanceolatum, less than half an inch in length (fig. 218). 



FlG. 221. Billiarzia h/rma- 

 tobia. Female in the gynaeco 

 phoral canal (c) of the male; 

 s', s", anterior and posterior 

 suckers. 



