Paludina Vivipara is a handsome snail, with a bronze tinted, globular shell; but Planorbis Corneus and carrinatus are still handsomer, having a spiral form, resembling the horn of a ram. These latter are to be trusted anywhere; they are good cleaners, and seldom attack the plants. Water snails breed rapidly in tanks, but the carp devour the young as fast as they appear; hence it is advisable to remove the spawn into jars containing healthy plants, such as Callitriche, in which they may remain for observation of growth, till stout enough to be committed to the tank.
PLANORBIS CORNEUS, PALUDINA VIVIPARA, LYMNEA STAGNALIS, UNIO PICTORUM, TUMIDUS, & ANODON CYGNEUS.
Among the bivalves, the fresh-water swan mussel, Anodon cygneus, and the Duck mussel, Unio pictorum, are interesting burrowers, and perform a great service in the tank. They act as scavengers, not by the process of eating off objectionable growths, as in the case of univalves, but by the straining off of matters held in suspension in the water, and filtering it in a pure state, by the mechanism of their syphons, and ciliated gills. It is very interesting to watch them thus engaged, and to note the force of the stream which they project from time to time.
The only creature of the insect kind that I can recommend for general adoption is the caddis worm, a comical and interesting creature, that can never mar the beauty of the tank. Half-a-dozen may be thrown in, and searched for occasionally—the search will always be well rewarded. When the cad closes his hybernacle, it will be desirable to remove it to a jar, to obtain a better opportunity of witnessing the transformation of the dormant worm into a four-winged fly of Stephens's family of Phryganea.
SELECTION OF STOCK.
The first thing to guard against is over-stocking, the common error of all beginners; taking large fish with small, I think about two or three to every gallon of water is the utmost that should be attempted. For a vessel of twelve gallons, I should recommend the following, as giving great variety, with considerable safety:—Six Prussian carp, of various sizes, one at least of five inches in length; two small Crucian carp; two small perch; two small loach; two tench, of five or six inches; six or eight minnows; one small eel; a dozen Planorbis corneus; half-a-dozen Paludina vivipara; three or four fresh-water mussels and a dozen of different sorts of newts.