FIELD BOOK OF PONDS AND STREAMS 



graphed through the sides. A reading glass is useful 



for observing the very small animals. 

 If the glass sides and the water become green, it means 



that too many algae are growing there. The aquarium 



should have less light, or there should be more snails 



to eat the algae. 

 If the gill-bearing animals keep going to the surface, they 



need more oxygen; there are too many animals and 



not enough plants. One small aquarium will support 



only a few animals. 

 Scum and bubbles on the top usually mean decaying 



material below. A greasy scum forming on the water 



very soon after it is put into the dish is a sign that it 



has been treated with alum. 



Vivaria. — Salamanders and frogs live well in terraria. 

 There are many ways of arranging these so that the animals 

 have water accessible and yet can climb out on the moist 

 earth. In making these as in making aquaria the best way 

 is to copy a small section of the animals' own home. 



Preserving specimens. — There are special methods by which 

 plant and animal specimens of different groups are best 

 preserved. These are suggested in discussions of several 

 different groups in this book, and are fully described in books 

 and papers listed in the Bibliography (p. 408). Animals are 

 generally preserved in alcohol or formalin, or by drying. 

 Books which give directions for preserving plants are listed 

 in the Bibliography. 



Killing animals. — Insects such as adult dragonflies and 

 moths should be killed in a cyanide bottle or with carbona 

 (p. 29) ; but aquatic larvae and nymphs and many other 

 small forms are best killed by dropping them directly into 

 alcohol. 



Fish, frogs, salamanders and the like should be chloro- 

 formed or killed by other methods which common sense may 



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