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commence with, but in no case must they be used unless adhering to the rocks or other substances on which they may be growing, as otherwise they would rapidly decompose and render the water quite unfit for animal life. Some taste too may be used in this matter, for glass bottles, old boots, and sheep's skulls, not unfrequently form the basis of attachment. Olive seaweeds cannot be recommended, although in an Aquarium, which we very lately had the pleasure of inspecting, the necklace shaped Hormosira Banksii, was the special delight of a dozen or more young and old Hippocampi or Sea horses (p. 37), which twisted their tails around its chain-like vesicles, evidently quite at home.
The best test by far of the tank being in a fit state for the introduction of stock is, when small crystal globules of oxygen appear on the plants. This happy condition being arrived at, we may proceed,—and yet on second thoughts it is as well to know something of the mode of respiration of the aquatic animals, since that will be of sure service in making a selection for the tank. The lower animals should be the first chosen, for one very good reason, that their respiration is far less active than in those of a higher organisation, and this applies pretty generally to all creatures inhabiting the watery element; in them respiration is performed by branchiæ or gills, varying much in form, and by the air with which the water is impregnated; thus in the Fishes we observe the mouth frequently opened to admit the water, which passing out through the gills serves to oxygenate their blood; a fish on
