2H() FRESH-WATER AQUARIA. 



constant. Dead animals should be removed, but decaying water 

 plants, though somewhat unsightly, are Bach excellent harbour and 

 food for microscopic creatures that they should rather be introduced 

 than removed. 



Occasionally, say once a year, in the late autumn or early spring, 

 it is well to have a thorough clean out. At these times all the live 

 stock which it is wished to preserve must be taken out, as also the 

 pieces of rock and plants, and so much of the water as can be taken 

 out in a clear state, drawn off with a syphon or otherwise carefully 

 removed into another vessel, so that it may be returned to the tank 

 together with the animals, plants, and stones after the cleaning 

 process has been gone through. The reason for this is that there will 

 probably be many germs in it, besides entomostraca, rotifers, and 

 other small organisms which it is desirable to retain. The whole of 

 the gravel and grit must be taken out and thoroughly washed. An 

 enormous quantity of dirt will be found mixed up with the stones. 

 This is chiefly the effete matter which, falling to be bottom, is 

 hidden among them — indeed, this is one of the uses they subserve. If 

 there are any snail eggs, vorticella, or other treasures attached to the 

 glass, they should be carefully stripped off, and although they cannot 

 be replaced in their original positions, they will, if returned to their 

 renovated home, continue to develop or form new colonies, as the case 

 may be. The glass, if there is any confervse on it, should be well 

 scrubbed with soap and warm water, taking care that all the sand and 

 grit is removed, otherwise there will be most likely a number of un- 

 desirable scratches to be seen upon it. 



In such an aquarium as has been described, almost any creature 

 that inhabits fresh-water can be kept in health with little trouble, 

 provided over-crowding does not take place ; but due care must be 

 exercised in the selection of its inhabitants. For instance, it will not 

 be wise to place fish in it, if it is wished to preserve minute forms of 

 life, these being their natural food. There should generally be a fair 

 number of snails present, as they greatly assist in keeping the vegeta- 

 tion within reasonable bounds, and being particularly fond of confervse 

 and very ravenous, the glass is usually kept fairly free from it, which, 

 as we have seen, is desirable. The best species are Lymruea stagnate, 

 Paludina vivipara, I', contecta, and any of the larger species of 

 Planorbis ; Planorbis corneus being the largest, is to be preferred] but 

 P. carinatus, /'. spirorbis, as also Lymnaa peregra and Bythinia 

 tentaculata are by no means unsightly, and are only inferior in point of 

 size to the others above mentioned. Snails are exceedingly inter- 

 esting objects in the very young state under the microscope, and the 

 pi-ocess of development in the egg can be better studied in those 

 of the fresh-water mollusca than perhaps in any other. Bythinia 

 tentaculata is the one of all others with which 1 ana acquainted that 

 arranges its eggs in the most convenient form for observation. The] 

 are placed in rows of two or three abreast and never on the top of one 

 another. They are. moreover, of a comparatively large size, and their 



