CHAPTER V. 



THE MOLLUSCA AND THE PISH. 



the plants are well established, 

 the water not forming the slightest 

 scum, and the tank furnished with 

 three or four common Pond Snails to 

 consume any decaying vegetation 

 that may occur, and keep down the 

 growth of the confervse which would 

 cover the glass, and render the spectacle of its 

 inmates imperfect, then the Aquarium is ready for 

 the reception of the fish ; which, from their grace- 

 ful forms and agile and pleasing movements, are 

 always the greatest favourites. 



Previously to describing the fish most suitable for 

 our purpose, we may derive some entertainment and 

 instruction from the examination of the family of 

 shell-bearing mollusca of our fresh waters, whose ser- 

 vices as scavengers are so important in the circle of 

 compensating principles, upon which the establish- 

 ment of the Aquarium is founded. The common Pond 



