A FROZEN WORLD 211 



in its normal position. It browses at times on the 

 submerged weeds in the pond ; but it has to come 

 to the surface at frequent intervals to breathe ; 

 though, in common with most aquatic air-breathers, 

 it can go a long time without a new store of oxy- 

 gen, like a man when he dives, or a duck or swan 

 when it feeds on the bottom of course to a much 

 greater degree, because the snail is cold-blooded ; 

 that is to say, in other words, needs much less 

 aeration. On a still evening in summer you will 

 often find the surface of the pond covered by 

 dozens of these pretty shells, each with its slimy 

 animal protruded, and each drinking in air at the 

 top by its open-mouthed lung-sac. 



In winter, however, as you see in No. 2, our 

 pond-snail retires to the mud at the bottom, and 

 there quietly sleeps away the cold season. Being 

 a cold-blooded gentleman, he hibernates easily, 

 and his snug nest in the ooze, where he buries 

 himself two or three inches deep, leaves him re- 

 latively little exposed to the attacks of enemies. 

 Indeed, since the whole pond is then sleeping and 

 hibernating together, there is small risk of assault 

 till spring comes round again. 



Now, it may sound odd at first hearing when I 

 tell you that what the animals thus do, the plants 

 do also. " What ? " you will say. " A plant move 

 bodily from the surface of the water and bury 

 itself in the mud ! It seems almost incredible." 

 But the accompanying illustrations of one such 

 plant, the curled pond-weed, will show you that 

 the aquatic weeds take just as good care of 



