All the species are usually infested with Cercariae and Rediae, the larval forms of Trematode parasites of vertebrates.
By a series of changes similar to those by which the primary larva arose from a segmented egg, so do these secondary larvae or "rediae" arise from the germ-cells or germ-balls within the sporocyst.
It escapes into the adjacent tissue and there gives rise either to one or more generations of rediae or at once to a new type of organism - the cercaria.
What determines the origin of the cercaria rather than a new generation of rediae is unknown.
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.