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. 



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 1879-80 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, i\ 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, yV 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. 



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