Notes on Animals in small Aquaria. 4539 



Memorandum 2. — Another very interesting experiment that I have 

 had progressing very successfully for some time past, is the preserving 

 sea water in a perfectly transparent and healthy state without the use 

 of vegetation of any kind, or, in some cases, even of a. scavengering 

 mollusk. The adoption of these experiments was in a great degree 

 forced upon me from circumstances which have been already pub- 

 lished. In the paper read before the Meeting of the British Associa- 

 tion at Hull, I stated that in consequence of the ravenous propensities 

 of the crabs and the varieties of rock-fish, I had been obliged to 

 establish several small imitation rock-pools, so as to separate these 

 various depredators from each other; and as some of these, the 

 blennies, also attacked the common periwinkle and other mollusks 

 which were employed as scavengers, the plant or vegetation conse- 

 quently became of little use, and was therefore omitted altogether 

 from the arrangement. It may be asked, then, how can the sea water 

 under such circumstances be possibly kept in a healthy state ? Why, 

 thus : by exposing a very extended surface of it to the action of the 

 air, and at the same time limiting its depth. The means that I have 

 been adopting for upwards of twelve months consist in the employ- 

 ment of shallow circular stone-ware pans of about eighteen inches 

 internal diameter by five inches deep; these are filled for about two 

 inches with water, the bottom is supplied with sand and shingle, and 

 numerous fragments of rock-work are arranged at the sides, some close 

 below the surface of the water, others rising in gentle slopes above, 

 and others again grouped to form cavities of retreat, so as to accord 

 with the habits of the crabs, blennies, &c, placed in them. The whole 

 is covered with a sheet of common window-glass, raised about one- 

 fourth of an inch from the edges of the pan by means of slips of wood, 

 so as to allow a free current of air over the surface of the water, and at 

 the same time impede the evaporation and prevent the greater part of 

 the dust and soot from settling on it. By this arrangement a very 

 extended surface of water is submitted to the oxidizing influence of 

 the air, and the fish and crabs, by their continual movements, cause 

 sufficient motion in the fluid to expose a fresh surface frequently to 

 its action, and thus keep up its aeration. But it must be borne in 

 mind, that the oxygenation of the water thus effected is a very 

 delicate equilibrium, and the maintenance of a healthy aeration is 

 liable to be disturbed by very slight interfering causes ; nor do I con- 

 ceive that this method would be applicable except to such marine 

 denizens as are either of such low organization as to require but little 

 aeration of the water, or to such as the crab tribe, the blennies, 



