The Pond Snails - Family Limnaeidae

Three stages of its life are passed, three generations of progeny developed, within the body of this long-suffering snail. From the liver the parasites pass out. Following their fateful programme they at once climb upon the stems of marsh grasses and encyst themselves for a dormant period that may never end.

Sheep are turned in autumn into pastures where grass is still green, and there is water. They eat the herbage, and a great many of the infested snails, besides the incysted forms of the parasite, put there as if for their especial benefit. Freed of their cysts and stimulated to wonderful activity in the stomachs of the sheep, the invaders pass to the liver, where as mature "liver flukes" they set up destructive changes by the symptoms of which the experienced farmer at once recognises the deadly disease called "rot." Three million sheep died from this cause alone in the winter of 1879-8o in England. The eggs produced in the liver of the sheep and excreted with the manure, await the coming of spring, when the deadly cycle of another generation begins.

Sheep kept away from marshy pastures escape the scourge.

Salt is a tested prophylactic.

L.

auricularis, Linn., a species easily distinguishable by its greatly expanded mouth, looks like a limpet with a spire attached on one side. This European species has an American counterpart in L. ampla, Mighels, which has a less exaggerated, but still very ample mouth, and a much inflated body whorl. Length, 1 inches. Eagle Lake, Maine.

L.

columella, Say, is oval with an acute elevated spire, and large oval aperture. Columella so narrowed that one can see the interior almost to the apex. Length, A- inch.

Habitat.—Miry places and stagnant waters. New England and Lake Superior to Georgia.

L.

palustris, Mull., oblong, with acuminate spire and much elongated and swollen body whorl, is a European species, found also from New England to the Pacific states. It is variable, averaging an inch in length.

292 The Pond Snails L. gracilis, Jay, has but four or five whorls, but is seven times as long as broad. It might be called "the needle pond snail." The aperture is elongately oval, and has no fold. Colour, white. Length, I inch.

Habitat.— Lake Champlain, Wisconsin and Ohio,