112 MOLLUSC A, INSECTS, ETC. 



entangles on the surface and carries below, so as to 

 fill the diving-bell-like nest it has spun with it, and 

 which is air-tight. This nest is filled by successive 

 journeys of the spider to the surface, to store up the 

 air for subsequent breathing. These air-nests may 

 be seen in most ponds, especially where there is an 

 abundance of Anacharis and water-crowfoot leaves. 

 There are several species of the order Arachnida 

 which live in water, but they are usually of the kind 

 we call "ticks." Some of them pass through very 

 interesting metamorphoses, being parasitic on plants 

 and insects alternately before they attain their fully 

 developed condition. 



It will be seen that the objects required for stocking 

 ordinary fresh-water tanks are not difficult to find ; 

 and, if proper precaution as to the habits and voracity 

 of the different kinds be taken, there need be little 

 fear as to ultimate and continued success. Every 

 aquarium, large or small, ought to be as perfect an 

 imitation of natural conditions as possible, and suc- 

 cess always depends on the degree to which this is 

 carried out. 



