THE ANIMALS OF THE FRESHWATER AQUARIUM 105 



attempt should not be made with forms captured in swift-running 

 streams. 



2. The Invertebrates of the Freshwater Aquarium. Passing now 

 to the invertebrates, we may begin with the MOLLUSCS, the highest 

 of the series. Of the three classes of Molluscs, the highest, 

 the cuttles, are purely marine, and the bivalves, as a general rule, 

 are so difficult to keep alive for any length of time in captivity 

 that they can hardly be attempted in the school aquarium. This 

 leaves us with the Gasteropods, or Univalves, including some 

 hardy and interesting animals which form useful inhabitants of 

 the aquarium. 



The available species of Gasteropods fall into two groups, 

 whose differences are interesting. Without stopping to consider 



FIG. 46. Freshwater snails : Paludina vivipara to right and Limncea stagnalis to left. 



the order to which these belong, we may note simply that one 

 group, represented by Paludina and its allies, are truly aquatic, 

 breathing by gills ; while in the other division, represented by 

 the very common Limncea, the animals, though living in water, 



