and cannot find it. Probably it has secreted itself in some corner or crevice, whence it will emerge in a day or two. Still such a circumstance should excite your vigilance.
Instruments.—For removing dead specimens or the like, a pewter spoon bent up to a right angle, with the shaft tied to a slender stick, is very useful. You can, if you please, make a more elegant affair of it. Two or three simple sticks or rods, some of them widened, spade-like, at the end, are also useful for pushing the specimens to any required point. And one or two small nets made by stretching a bit of lace or muslin over a ring of wire, fastened to a rod, will serve to catch and lift out such animals as you wish to transfer, for examination, or any other purpose, to another vessel. As a general rule, however, they should be disturbed as little as possible, and never handled.
Artificial aeration.—Although living and healthy plants will educe and throw off, under the influence of light, oxygen, in sufficient quantity to maintain in health a given number of animals, yet the artificial admixture of atmospheric air with the water may be employed as a valuable auxiliary. I have used it with marked benefit; often having revived animals thereby, which, from the exhaustion of the water, were apparently in a dying state. Its utility as a means of maintaining the purity of the water is still more obvious; since, as I have more that once had occasion to observe, it is by the frequent and successive presentation of the particles of water to the air, that the animal excretions which they hold in suspension,
