264 POPULAR HISTORY OF THE AQUARIUM. 



foot; lastly, the end of it pulls the horny lid into its 

 place. Troclii with conical shells have somewhat similar 

 habits, and their tongues are similarly formed and similarly 

 used. 



LlMX.'EA STAGXALIS, AND OTHER WaTER-SnAILS. 



T\liat L'lttorina does for Marine, LlmncBa does for Fresh- 

 water Aquaria. The latter may be seen in the tanks, mow- 

 ing away at the microscopic vegetation which would other- 

 wise obstruct our view. The shell of Limncea stagnalis is 

 elegantly formed, transparent, and obliquely conical, while 

 those of PlanorblSj another genus of Water-Snails, is a flat 

 spiral coil; but the animals are very much alike in nature 

 and habits. Paludlna, which has an oval shell of rounded 

 whorls, is speckled all over the body with minute dots of 

 gold. 



Although the "Water-Snails are in some sense useful as 

 scavengers, they are also mischievous as devourers of use- 

 ful and sightly vegetation, biting through the leaves of 

 VaUisrie7'ia, and other plants, as remorselessly as Garden- 

 Snails make holes in cabbages. The smaller species, such 

 as Limnrea auncidata and L. gbif'niosa, Phj/sa foufinal'is 

 and Blfh'mia ientaculataj are the most useful and least harm- 



