LIMNEAS. 



99 



fresh-water fishes and amphibians with which the 

 ordinary aquarium may be stocked ; and now briefly 

 refer to other species not belonging to these orders, 

 which are equally common in all our ponds and 

 tarns, and equally interesting and animated when 

 transferred to the aquarium. First of all we may 

 notice those commoner species of water snails, whose 

 hardiness and voracity prove of great service in 

 keeping down the impalpable green algai, which will 

 develop even in the best managed aquarium. Of 

 these none are more abundant 



rig. 46. 



than the Limnacece, and of the 

 eight British species included in 

 the order, perhaps the best is 

 Limnea stagnalis. It will crawl 

 over the inner surface of the 

 glass, and keep it as clean as 

 if well dusted. Occasionally it 

 may be seen floating, and then 

 before it descends to the bottom 

 of the tank, it utters a percep- 

 tible sound, caused by disengag- 

 ing the air from its pouch, which 

 had kept it buoyant. This 

 species is the handsomest we Lymnea stagnalis. 

 have, and the young shells are especially graceful 

 and slender. Limnea auricitlaria is a much smaller 

 shell, with a larger body-whorl, the outer lip of which 

 is reflected. Although not so common as L. 



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