ANIMALS FOR. 



2. Mollusca. 



The ocean is the great home of the Mollusca, but 

 some of them live on land, as our familiar slugs and 

 snails ; and others live in fresh waters. These may 

 be collected for the fresh Aqua vivarium. We have 

 before pointed out their office as scavengers, and a 

 few forms will be introduced to the reader. The Mol- 

 lusca are divided into two great families, one gene- 

 rally characterized by having one shell (Gasteropoda), 

 the other by having two shells (Conchifera). The 

 first produces the most numerous and more common 

 examples. The Coil-Shells (Pla- 

 norbis) are the prettiest and most 

 useful of these animals. The 

 Margined Coil-Shell (Planorbis 

 marginatus), Fig. 19, is found 

 very commonly in stagnant waters 

 and slow rivers ; and from its 

 habit of eating the Confervse, and 

 avoiding the higher plants, is one 

 of the most useful of the Mollusca 

 in the Aquavivarium. 



The Mud-Shells (Limneus) are a great contrast 

 in form to the Coil-Shells, and the 

 smaller species, as the Marsh Mud- 

 Shell (Limneus pereger), Fig. 20, may 

 be safely introduced into our fresh- 

 water collections. Care must, how- 

 ever, be taken about the large Lake 

 Mud-Shell (Limneus stagnalis), Fig. 21, 

 which has an appetite proportionate 

 to its size, and prefers above everything 

 a salad of Yalisneria. Where the 

 vegetation in a jar or tank is very 



FIG. 19. 



FIG. 20. 



