268 MOLLUSOA. 



found together, and they are most prolific. One may find them 

 at all times of the year. The shells are elongated and conically 

 oval, the spire heing produced, while the snail's body is twisted 

 up in the spire of the shell They have a prominent head and 

 an oblong foot, notched in front and rounded behind. The 

 shells are extremely variable, one species (Limnoeus pereger) 

 having at least fourteen varieties. In this country the two 

 species important to us are L. truncatulus and L. pereger. Both 

 are hosts of the liver-flukes (Distomidm), especially the all- 

 important liver-fluke (Distomum hepaticum) which causes the 

 " liver-rot " in sheep. 



L. trunaatulUs (fig. 140, a) is a pale ashy-grey turreted shell 

 with five or six whorls, the last whorl being large and expanded, 

 and occupying about three-fifths of the 

 shell; the spire tapers to a very fine 

 point. It is found nearly all over 

 Britain and Europe, Afghanistan, Mo- 

 rocco, Algeria, the Canary Islands, &c. 

 This snail lays its ova or spawn upon 

 Pig. uo. -Water-snails ^^^ around ponds, ditches, and 

 (Limnceu^). streams. Each snail lays about 1500 



L. pereger. Twice nat. size! OVa, m batches of 30 to 100. They 



are deposited in strips of some gelatin- 

 ous substance. Incubation takes place in about two weeks. 

 This is the chief host of the liver-fluke. 



L. pereger (fig. 140, b), the other fresh- water snail, is almost 

 transparent yellowish - brown, and spirally striate, with five 

 whorls. This species often wanders far from water, and may be 

 found crawling up willow-trees and in damp meadows. They 

 are carnivorous in habits. Although the embryos of the liver- 

 fluke are found in this species, it seems it cannot complete its 

 changes in it. Were there no Limnseidee there would be no 

 liver-rot in our flocks ; but it is extremely difficult to destroy 

 these molluscs. There is no doubt that the suggestion made by 

 Miss Ormerod of clearing out shallow pools of weeds, and re- 



