FRESII-WATEK FISH AND MOLLUSCA. 59 



your shell-covered mollusca — the snails, for instance — come 

 additionally into play. They act as natural scavengers, 

 and soon devour the obstruction, if not entirely too abun- 

 dant. In the latter case, the tank must be placed in a 

 comparatively dark place for a few days, when all the 

 superfluous matter will disappear. 



As variety is of some consequence, it may be as well to 

 have more than one kind of Mollusca to enact their 

 humble but valuable role in the Aquarium. Considerable 

 caution, however, must be exercised in making a selection. 

 Some of the Mollusca prefer to feed altogether on the 

 confervce and on the vegetable decay, while some are of 

 so destructive a nature, that they demolish indiscriminately 

 the confervoid growth and the large favorites of the 

 Aquarium. The latter should, of course, be avoided. 

 They may be interesting to contemplate, but they are 

 expensive to keep. 



Of the most useful and harmless Mollusca, we should 

 recommend the common Marsh Shell {Faludina vivipara) 

 the Trumpet Snails {Planorhis corneus and P. carinatus^ 

 and the snail known as the Gluiinosa. The first has a 

 light-colored shell, with thin red lines. Occasionally one 

 has a shell of a greenish-brown, marked transversely with 

 brownish red bands. The second are flattened in shape, 

 sometimes angular and sometimes keeled. They may be 

 found in low marshes and ditches, as well as in ponds. 

 The last mentioned is a smaller snail, but active and in^- 

 dustrious. 



