10 POPULAR HISTORY OF THE AQUAEIUM. 



tended to represent. Some marine animals burrow, and 

 should therefore have a bed of suitable material to gratify 

 their propensity. It is best to use sea-sand for sea-water, 

 and river-sand for fresh-water. Then a few clean bright 

 pebbles give a pleasant appearance, and afford shelter for 

 minute animals. In marine tanks only is rockwork ad- 

 missible. This must be made according to the taste of 

 the proprietor. Pieces of natural rock, or large stones 

 cemented together, and shaped by any cement that hardens 

 under water, may be used ; they should be set up with 

 projecting ledges, and forming hollows and arches, so as 

 to give shelter to those animals which seek it, and present 

 a variation in the position of various growths. Too much 

 formality should be avoided ; but, as a rule, the tank would 

 best represent a shore-grotto, if the larger rockwork were 

 placed at the sides and in a half-circle at the back. In a 

 fresh-water tank, a few large stones laid down on the 

 pebbles and sand is all that will be desirable. The orna- 

 mentation in this case must consist principally in the 

 plants. 



Aspect and Admission of Light. — In visiting the Zoolo- 

 gical collection the eye is pained in some instances by the 

 foul and stagnant appearance of some of the tanks. A 



