INTRODUCTION. 5 



represent the primary idea of a Water Yivarium^ yet tlie 

 experiments are not all confined to those organizations 

 which keep within shallow waters : even denizens of the deep 

 may be provided with such accommodation as will almost 

 make them feeL themselves at hom.e. 



Xor is it only for aynnsement that such parlour oceans 

 and lakes are prepared and stocked ; they are invaluable 

 as a means of instniciion. The natures of living beings 

 can never be thoroughly known but by tlieir habits ; their 

 habits cannot be well understood unless closely and con- 

 tinuously observed ; and if we cannot go down among 

 moUusca, Crustacea, and zoophytes to examine them in their 

 native haunts, we can now bring them up to us, to be 

 studied in nearly similar conditions. 



The principle upon which \Yater Yivaria are constructed 

 and maintained consists mainly in balancing animal by 

 vegetable life, thus : — If a few fish were confined in a 

 vessel of water, and the water remained unchanged, they 

 would soon droop and die. The water would not sustain 

 life after the animals had deprived it of its oxygen by 

 passing it through their gills in breathing. If, by means 

 of a fountain-jet, the water can be raised up and returned 

 through the air into the tank, or, in other words, be 



