202 THE ZOOLOGIST. 



and it is not unusual to see several together during the season 

 of reproduction. All the members of the genus Limnceus 

 frequent shallow and still waters or damp fields, and are most 

 prolific and gregarious. They may be found at nearly all times 

 of the year. The shells of these water-snails are elongated or 

 conically oval, the spire usually produced, dextral, or turning 

 from left to right. The body of the snail is twisted in the spiral 

 and long ; the head is very prominent ; tentacles short, triangular, 

 and depressed ; the foot oblong, notched in front and rounded 

 behind. Some species have the shell enveloped by folds of the 

 mantle. Like many Mollusca, the Limnceidce are extremely 

 variable, one common English species, Limncea peregra, having at 

 least fourteen well-marked varieties. 



In this country, and in Europe generally, the two species of 

 economic importance are L. truncatula (=minuta of Draparnaud) 

 and L. peregra. Both these species, which are cosmopolitan, 

 are the hosts of the flukes (Distomidce) , especially the all-important 

 liver-fluke (Distomum hepaticmn), which causes the "liver-rot" in 

 sheep. The eggs of the fluke are voided in the sheep's excreta 

 on to the grass and around the ditches and ponds. The little 

 infuseriform embryos coming from them pass, if they can, into the 

 water, and are very active ; if they meet with one of these two 

 water-snails within twenty-four hours they pass into it, and there 

 they remain, giving rise to other stages of their life-history (redia 

 and sporocysts). These give rise to another stage, the so-called 

 Cercarice, which leave the mollusc and become free-swimming. 

 They then encyst on the grass near the water, and, being eaten 

 by the sheep, turn into the fluke. If these embryos did not 

 meet with a Limncea they would die, and the fluke would dis- 

 appear from our sheep. Thus the importance of these water- 

 snails can be readily seen. By far the greater number of these 

 embryos enter L. truncatula of Miiller, the L. minuta of Drapar- 

 naud) which is described below. 



L. truncatula. — Shell oblong-conic, turreted, shiny, pale, 

 horny, ashy grey, five to six whorls, which are rounded and 

 convex, but slightly compressed in the middle, last whorl large 

 and expanded, occupying about three-fifths of shell ; spire of shell 

 tapers to a very fine point; mouth oval; outer lip sharp, inner 

 lip continuous with it and reflected on the columella; height 

 about 8 mm., diameter 8 to 5 mm. This species is found over 



