THE FRESH-WATEE AQUARIUM. 75 



it clean, yet I assert that though they may not be among 

 the most active of the creatures that we place in our 

 tank, still for the beauty of the forms of their shells, if 

 not from their curious mode of progression over the glass, 

 and floating along the surface of the water, as well as for 

 the purpose of watching them bring up their broods of 

 minute snails, which, in time, serve as food for our fish, 

 they are to be viewed not merely as "necessary evils," 

 but as good, interesting, and pretty citizens of our world 

 in miniature. 



The greatest in quantity and beauty among the Mol- 

 lusca are undoubtedly those dwelling in the sea ; but some 

 are inhabitants of fresh water, as the familiar water-snail 

 that we find in the mill-dams and ponds, whilst others 

 inhabit the land, as the common garden slug. The Mol- 

 lusca form a class of animals that bear shells on their 

 backs and whose coverings we pick up along the beach, 

 and which constitute some of the first objects of natural 

 history that we learn to admire for their beauty of form 

 and the varied tiuts that they present. The shells of dif- 

 ferent genera of Mollusca are of varied form and texture ; 

 thus, those of the salt-water Mollusca arc, in general, 

 hard and strong, whilst the shells of the fresh-water snails 

 are thin and more of the consistence of horn than what 

 we understand as shells. There are some of the mollusca, 

 however, that seemingly do not present us with any 

 shells at all, as the garden-slug ; but they have shells, 

 though they are small and hidden in the substance of 

 their bodies and do not serve as a means of defence, as 

 those of the well known oyster or clam do to them. 



