3744 The Zoologist — November, 1873. 



consumed by crabs or other scavengers is distributed in every direction as 

 fast as it becomes decomposed and assumes the gaseous form, and a supply 

 of pure water immediately takes its place, so that the plants and animals in 

 the neighbourhood are not exposed to any hurtful influence arising from it, 

 as is the case when the decaying matter gives off its deleterious gases in the 

 motionless water of the aquarium. In large estabhshments this movement 

 of the water may be produced without much difficulty by the aid of gutta- 

 percha pipes and a small reservoir or supply tank from which the water may 

 flow into the aquarium and the waste be pumped back to continue the 

 circulation ; for it is not necessary that the water should be renewed. Atiy 

 loss that may take place by evaporation should be made good by the addition 

 of pmre fresh water ; but the yreat bulk of the sea-water icill last for years if 

 it is kept well aerated ; indeed, there is no reason for its ever being unfit for 

 use. The salts held in solution retain their properties for an indefinite time, 

 as far as is known ; and as the pure water is all that is lost by evaporation, 

 that alone requires replenishing in order to preserve the proper specific 

 gravity or density of the whole. In small tanks the water may be tem- 

 porarily agitated by the use of a convenient kind of force-pump, adopted by 

 Mr. W. A. Lloyd ; but unless there is a special supply tank placed on a 

 higher level than the aquarium, so as to ensure a continuous stream, the 

 movement of the water cannot be easily maintained. Prawns and fishes are 

 also useful, to a certain extent, in causing a motion of the water, but they 

 have too little power to produce anything like the continued washing of the 

 sea which the Actinife are accustomed to ; for it will be remembered that 

 these animals, being attached, are exposed to the recoil of the water from the 

 resistance of the rock as well as to the influence of the tidal current ; and 

 this double motion of the water produces the wash which gives such a life- 

 like appearance to everything growing within its reach. Animals subjected 

 to this peculiar movement of the sea display a vigour almost unknown in the 

 usually quiet waters of the aquarium. The Actinia? attach themselves 

 firmly in sheltered crevices and expand their flower-like disks to the ever- 

 changing water around them, every wave brings a fresh supply of food 

 within their reach, and their bodies are kept clean by the motion of the 

 water. Fishes, and other swimming animals, also seem to enjoy the con- 

 tinual struggle necessary to prevent their being carried away by the stream, 

 and thus all their vital powers being called into action, everything presents 

 the appearance of health and animation. Such are the results we must 

 endeavour to produce in the aquarium, and we can only do so effectually by 

 imitating, as far as possible, the means employed by Nature. In cases 

 where it is inconvenient or difficult to establish a constant change of water, 

 as in small tanks, great advantage may be derived by occasionally drawing 

 off the water,- and in this manner placing the animals ui the condition to 

 which they are accustomed when the tide is out. Of course this treatment 



