AERATION. 271 



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 present- 

 ation of the particles of water to the cir, that the 

 animal excretions which they hold in suspension, 



