GO THE SEA SHORE 



Concerning the first of these we must caution the reader against 

 the common error of overcrowding the aquarium with animals. It 

 must be remembered that almost all marine animals obtain the 

 oxygen gas required for purposes of respiration from the air dissolved 

 in the water. Now, atmospheric air is only very slightly soluble in 

 water, and hence we can never have an abundant supply in the 

 water of an aquarium at any one time. If a number of animals be 

 placed in any ordinary indoor aquarium, they very soon use up 

 the dissolved oxygen ; and, if no means have been taken to replace 

 the loss, the animals die, and their dead bodies soon begin to 

 putrefy and saturate the water with the poisonous products of 

 decomposition. 



It is probably well known to the reader that a large proportion 

 of the oxygen absorbed by the respiratory organs of animals is 

 converted by combination of carbon into carbonic acid gas within 

 their bodies, and that this gas is given back into the water where 

 it dissolves, thus taking the place of the oxygen used in its forma- 

 tion. 



If, then, an aquarium of any kind is to be a success, some 

 means must be taken to keep the water constantly supplied with 

 fresh oxygen quite as rapidly as it is consumed, and this can be 

 done satisfactorily by the introduction of a proportionate quantity 

 of suitable living weeds, providing there is not too much animal 

 life present. 



The majority of living plants require carbonic acid gas as a food, 

 and, under the influence of light, decompose this gas, liberating the 

 oxygen it contained. This is true of many of our common sea- 

 weeds, and thus it is possible to establish in a salt-water aquarium 

 such a balance of animal and vegetable life that the water is main- 

 tained in its normal condition, the carbonic acid gas being absorbed 

 by the plants as fast as it is excreted by animals, and oxygen 

 supplied by the plants as rapidly as it is consumed by the animals. 



This condition, however, is more difficult to obtain in a salt-water 

 aquarium than in one containing fresh-water life, partly because, 

 generally speaking, the sea-weeds do not supply oxygen to the water 

 as rapidly as do the plants of our ponds and streams, and partly 

 because of the difficulties attending the successful growth of sea- 

 weeds in artificial aquaria Thus it is usually necessary to adopt 

 some means of mechanically aerating the water; but, for the 

 present, we shall consider the sea-weeds only, leaving the mechani- 

 cal methods of aerating the water for a later portion of this chapter. 



