All the species are usually infested with Cercariae and Rediae, the larval forms of Trematode parasites of vertebrates.
D, An adult redia, containing a daughter-redia, two almost mature cercariae, and germs. E, A free cercaria.
The number of cercariae produced by the pullulating rediae in a single water-snail is immense, and as they are emitted at a given period or a few successive periods, the snail at these times appears enclosed in a cloud of whitish flocculent matter.
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