SCAVENGER DEPARTMENT. 21 



shingle, and numerous fragments of rock-work may 

 be 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 Crabs, Blennies, &c., 

 which may be placed in them. The whole can be 

 covered with a sheet of common window-glass raised 

 about a quarter of an inch from the edges of the pan 

 by means of slips of wood, so as to allow a free cur- 

 rent 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 absorption of 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 oxygenization of 

 water thus effected is a very delicate equilibrium, 

 and the maintenance of a healthy aeration is liable 

 to be disturbed by very slight intervening causes; 

 indeed, practically, this method is only applicable to 

 such marine denizens as require but little aeration of 

 the medium they respire, or to such as the Crab tribe, 

 the Blennies, Cotties, Gobies, and creatures which de- 

 light in very shallow water, or which have the power 

 of climbing out of their liquid element. 



Having stocked our tank with appropriate vegeta- 

 tion in sufficient abundance, it only remains to re- 

 present the scavenger department by introducing a 

 few hungry mouths to browse upon the rank luxuri- 

 ance of vegetable growth, and to keep in check the 



