THE AQUARIUM 75 



in the Pennine Range. This limestone was formed at the bottom 

 of the sea, and the carbonic acid which it contains was removed 

 by shell-forming animals from the water. The same thing is 

 true of the chalk of the South of England. Though such shell- 

 forming animals do not, of course, produce oxygen like the green 

 plants, yet they have a negatively purifying action in removing 

 the carbonic acid. The point is that in nature the water is 

 purified and aerated by agents which are not active in the aquarium, 

 and that therefore the balance in the latter between the plants 

 and animals can never be anything but precarious. The result 

 is that in all large aquariums some mechanical agency is added 

 to the purifying action of plants, or even substituted for this. 

 In saltwater aquaria, for instance, sea water is pumped through 

 the tanks containing the animals, and in freshwater aquaria 

 water is introduced under pressure, so that in its rush it carries 

 abundant oxygen with it. The conditions of the school aquarium 

 prohibit any elaborate mechanism for aeration, but if it be poss- 

 ible, at least temporarily, to place the vessel beneath the drip 

 of a tap, or even beneath the drip of a rainwater barrel in the 

 open, the overflow being of course provided for, the more active 

 aquatic animals, like fishes, which for the most part demand 

 much oxygen, and will not tolerate stagnant water, can often 

 be kept alive for some time. In the rare cases where abundance 

 of water is available, and the somewhat troublesome business 

 of arranging for an overflow possible, various ingenious arrange- 

 ments of fountains have been suggested. For details of these 

 reference should be made to the books mentioned at the end of 

 this chapter. In the general case, however, the teacher who 

 wishes to keep in captivity and observe a considerable variety 

 of animals will do well to frankly face the necessity for the periodic 

 renewal of the water of the aquarium, and therefore for the 

 reduction of this to such a degree of simplicity as to make the 

 renewal, combined with cleansing of the vessel and its contents, 

 a relatively simple matter. 



It may be said, indeed, that if the object is observation of 

 habits, the simpler the aquarium the better. As to its form, a 

 large pie-dish forms a capital receptacle for a few special forms, 

 whether marine or fresh water. The glass bowls sold for keeping 



