THE POND SNAILS - FAMILY LIMNAEIDAE. Shell fragile, variable in form, horn-coloured, usually with an oblique fold on the columella; outer lip simple, acute. Head with broad muzzle, dilated at the end; mouth with one or more jaws, radula armed with numerous quadrate teeth; tentacles flattened, eyes sessile at inner bases of tentacles; foot flat; res piratory orifice on right side. Sexes united.
Fresh water mollusks, which come to the surface to breathe fresh air, and feed on confervx and other aquatic plants and small animal forms. They form an important staple in the diet of fishes, frogs, toads and birds, including coots, rails and others that frequent ponds and streams. In winter they bury themselves in mud. Distribution universal.
Genus LIMNJEA, Linn.
Shell dextrally spiral, oblong, translucent, with thin epi dermis, last whorl large; aperture large, roundish; lip simple; columella with one oblique fold. Tentacles flattened, triangular; mantle edge thickened; foot short, rounded. A world-wide genus, preferring the north temperate zone. In North America it reaches its maximum size in the region of the Great Lakes.
Left-handed Linmxas occur in the Sandwich Islands and New Zealand. The genus is represented in hot sulphur springs in Iceland, and in Lake Geneva at a depth of Soo feet. A species is found in Thibet at an altitude of 14,000 feet. Another creeps over ice fields in northern Asia, and is frozen in solid blocks of ice ten months in the year.
The Great Pond Snail (L. stagnalis, Linn.) has spread from Europe throughout the northern hemisphere. In North America it is found in still water and in quiet streams from Green land to Alaska and south to Texas. In winter it is frequently seen creeping along the under side of the icy crust of a brook.
290 The Pond Snails The spire of this snail tapers above the large body whorl to a needle-pointed apex. The whorls are rounded and separated by a deep suture. The outer lip is thin and does not flare. The columella is somewhat callous, and bears a strong oblique fold. The yellowish brown surface is coated with an epidermis, which is usually worn off on the spire, and the shell substance corroded by carbonic acid in the water.
The body is yellowish gray; the broad foot is edged with yellow. The large square head bears pointed tentacles that broaden toward their bases. The largest specimens are two
inches long. Half-grown shells are transparent and more slender than the adults.
L. stagnalis is a general feeder, exhibiting decided carni vorous tastes, though it is generally rated a vegetarian. In the fresh-water aquarium this bloodthirsty snail attacks newts and sticklebacks and contentedly picks their bones. Larvae of water beetles and other insects have also been its victims, and even its own young are not exempt. These depredations are not justified on the grounds of hunger. Plenty of its favourite vegetable food was in the jar. It is strange that a snail is able to capture crea tures so strong, so agile and in some instances so well armed. In a pond in England an old newspaper was found to be covered with snails which were hungrily feeding upon its substance.
The eggs are laid in cylindrical masses, counting from fifty to one hundred or more. The young grow most lustily in large ponds and streams. It is a fact well established by scientific tests that "rate of development and ultimate size attained are in direct proportion to the volume of water in which the individuals have lived." It is not surprising that so susceptible and so cosmopolitan a species should exhibit infinite variations.
The Dwarfed Limnma (L. truncatula, Mull.) means no harm at all, but it probably works more harm to the human race than any other mollusk. And this by being the innocent victim of one of Nature's cruel conspiracies, which gives the victory to a disgusting parasite.
Our little truncatula has a pointed, conical shell not over half an inch high, with shiny, deeply sutured whorls, and a distinct umbilicus. It scrapes the yellow-green alga; from the surface of ditches and ponds, "askin' nothin' f 'm nobody." But around it swim the ciliated embryos of Fasciola hepatica: a single adult 291 The Pond Snails lays half a million eggs. A strange instinct leads these micro scopic "whirling dervishes" to climb into the air chambers of the snails. They stab the flesh with horns like a unicorn's, that work in, being barbed. The scramble is justified. If the embryo has not found its host in eight hours after hatching, it dies. No other species of snail is attacked.