SALT-WATER AQUARIA. 115 



it can, if necessary, be renewed from the sea ; the mass of the water in the reserve tanks is 

 small as compared with that in the show tanks, and aeration is effected by pumping air 

 into the tanks through tubes of large diameter. At the Crystal Palace aquarium a constant 

 circulation is maintained from one tank to another ; the bulk of water in the reservoir is 

 five times as much as that in the show tanks, while aeration is accomplished by carrying 

 a main over their entire length, from which, under pressure, a small stream of water pours 

 from a tap into each, breaking the surface of the water, and carrying down to the 

 bottom of the tanks and distributing over the body of their contents myriads of minute 

 bubbles of air, which present an enormous oxydising surface to the water, rendering it 

 bright and sparkling. It does not answer to change the water too constantly, while 

 some obnoxious specimens, like the flat-fish, foul it greatly, the remedy for which is 

 found in putting animals in who in the economy of Nature act as scavengers. Various 

 small animals have to be supplied as food for the larger ones. " As the animal life and 

 vegetable life mutually support each other, the kind of material necessary for maintaining 

 the ' compensating system' must be watchfully supplied. Mr. W. R. Hughes, of Birming- 

 ham, recommends the growth of sea lettuce ( Alva latissima) in tanks, as suitable both for 

 oxygenating the water and for food for the fishes ; the stock plants being introduced in the 

 autumn months, when they are loaded with spores." The writer of the article in the 

 " Encyclopsedia Britannica," from which most of the above is taken, ventures to hope that 

 the aquarium may become useful in a practical sense, and may determine many questions in 

 regard to fish life and growth concerning which we are ignorant to-day. " It would," 

 says he, "tend to the better regulation of our fisheries and to the augmentation of our 

 food supplies, if we knew as much about the herring or the haddock as we do about the 

 salmon." It is well known that fish, valuable as food, are too often captured at improper 

 seasons and in a wasteful manner. 



Passing on to higher forms of animal life, the polyps and acalephse of the older 

 authors, now classified as the Ccelenterata, we find creatures of a superior organisation 

 to those lately under notice. Regarded generally, their bodies are soft and gelatinous, 

 they possess alimentary canals and digestive apparatus, and in nearly all cases the sexes 

 are separate, generation being sometimes sexual and sometimes by gemmation or budding. 

 This brief introduction to the subject must be taken only in a general, not a special 

 sense, for there are numerous exceptions to be found among the animals classified as 

 Coelenterata. 



"The sub-kingdom Ccelenterata naturally divides itself into two groups that of the 

 Hi/drozoa, and that of the Actmozoa. The fresh-water hydra will serve as an example of 

 the first, and the common sea-anemone of the second group. The essential difference 

 between the two is, that in the former the stomachal cavity is not separated from the 

 general cavity of the body, and the reproductive buds are external ; while in the latter the 

 stomachal cavity is let down, as it were, as a partially-closed sac, into the general cavity of 

 the body, and the reproductive buds make their appearance between the walls of the general 

 cavity of the alimentary or stomachal sac, and consequently internally. But in both there 

 is a free communication between these two cavities a communication obvious in the 

 Hydrozoa, and which may be often verified in the case of the sea-anemones, by the young 



