•240 



PLATHELMINTIIES 



they possess either the oral sueker ahme (MiuiosUviiinn) or tliis and a 

 seeond \'enlral sucker, and onl\- rarely other attaehiiij; apparatus. Tliey 

 are markedly separated from the Tolystomes by tJieir life liislory. The 

 alternation of hosts necessitated by the endoparasitie life is complicated 

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

 hence 'Digenea.' To illustrate this the history of Distoiniim hcpal'u'tim, 

 the hver lluke of the sheep is chosen ^I'lg. 220). 



The eggs leave the maternal uterus at the beginning of embryonic 

 development, pass down tlie bile ducts and thence liv the intestine to the 

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

 the ciliated larva {iniracidiiim, .1) escapes liy lifting of the lid of tJie shell. 

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

 grows into a sporocyst [B). 'The sporocyst, a muscular sac with proto- 

 iiephridia Init lacking all other organs, produces in its interior eggs wluch 

 develop into a second reproductive sac, the rcdia {D). These are dis- 

 tinguished from the sporocysts 1)y 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 arc cither 

 ccrcaria- {F), or another generation of redi;\; 

 may follow before the ccrcaria' appear. 

 The cercarix are adapted for an aquatic 

 life, since each has, besides the characteristic 

 organs of a llisloDiuDi (genitalia excepted), 

 a strongly vibratile tail. The ccrcaria' 

 escape froni the snail, swim about in the 

 water until the tail drops olT, when they 

 encyst on water plants. \\'hen these en- 

 cysted young are eaten bv sheep along with 

 tire vegetation, infection follows. 



In general it can only be said of (he life 

 history of other Trenialoda (hat the miracidia 

 must penetrate a mollusc, and that the dilTerent 

 S|)ecies luwe many modifications. Hcsl known 

 are the follnwing: Dhtomiiiii (FasiioUiria) 

 Itipiiliniiii, (he liver fluke (fig. 217), about the 

 si/e and shape nf a innn]ikin-seed, li\es in (he 

 bile-duets of shee]i, cows, |iigs, etc., and rarely 

 of man. It causes a disease known as 'li\'er rol,' generally resulting in death. 

 This history shows why she^p pastured in moist places are subieet to (he 

 disease, and why wet seasons arc times of cpideniies. Thus in the rainy ^-ear 

 of 1S30 about one and a half millions of sheep were killed in laigland; in 1S12, 

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

 by D. laiu-colalitiu, less than half an inch in length (fig. 21S). 



Fig. 221. — Billiarzia lurma- 

 tnbia. Female in the g\'n;eco- 

 phoral canal {<-) of (ho male; 

 s', s", anterior and inslerier 

 suckers. 



