. A manual of zoology. Zoology. •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
. A manual of zoology. Zoology. •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 fol
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
