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.
The attention of birds is speedily attracted to the snail by this appearance and by the peculiar movements which the worm executes, and the passage of the parasite into its final host is advantageously effected.
The mound builders, Pueblo tribes, middle Americans and Peruvians, were potters of many schools; gorgeous colour fascinated the Amazonians, the Patagonians delighted in skins, and even the Fuegians saw beauty in the pretty snail shells of their desolate island shores.
In many cases it appears that only the brilliantly coloured tentacle is pecked off by the bird, and as the snail can easily regenerate a new one, this in turn becomes infected by a fresh branch of the sporocyst ramifying through the snail and thus a new supply of larvae is speedily provided (Heckert).
The Romans are credited with having purposely introduced the edible snail (Helix pomatia) into England, and the common garden snail and slugs (Helix aspersa, Limax agrestis and Anion hortensis) have been unwittingly established in New Zealand.
The generative apparatus of the snail (Helix) may serve as an example of the hermaphrodite apparatus common to the Pulmonata and Opisthobranchia (fig.
The liver-fluke (Distomum hepaticum) passes through its larval stages in the water snail Limnaea truncatula in Europe; in L.
In this duct and sac the spermatophores received in copulation from another snail are lodged.