LAKES AND TARNS 145 



thick-shelled " winter eggs " which are able to resist 

 difficult circumstances such as severe cold or great 

 drought. Some of the beautiful little animals known 

 as Polyzoa, which live in colonies, form peculiar ex- 

 ternal buds called " hibernacula " and still more 

 peculiar internal buds called " statoblasts," which are 

 very resistant to cold. In some of the freshwater 

 Polyzoa, like Paludicella, the colony dies down in the 

 winter, but the hibernacula continue the life and start 

 fresh colonies in spring. In others, like Plumatella 

 and Cristatella, the statoblasts float away, and experi- 

 ments have shown that their power of developing in 

 spring is actually helped by subjection to a certain 

 amount of frost. Life seems often to be the better 

 for a see-saw between activity and rest, between 

 vigorous agency and lying low. 



The small creatures of the tarn, higher in the scale 

 of being than Hydra, are Planarian Worms, Thread- 

 worms, freshwater Worms with red blood, a few 

 Leeches, some Rotifers or Wheel Animalcules, and 

 other minute multicellular animals, numerous small 

 Crustaceans, a few Water Snails, and a considerable 

 number of insect larvae, which become Midges and 

 Gnats, Water Beetles and Caddis Flies, and even 

 Dragon Flies in the summer season. We wish to say 

 a little about the last, which are often to be seen 

 hawking for smaller insects over the surface of the 

 tarn. 



For many reasons Dragon Flies command our 

 admiration. The flight is powerful, sometimes rising 

 to sixty miles an hour; it is varied, now skimming the 

 water, again soaring in a spiral, and again sailing 

 over the moor; the two pairs of wings work inde- 



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