WATER PLANTS AND ANIMALS 



Water and living things. — Plants and animals grow in every 

 brook and pond. The first ones lived in the water and their 

 descendants still dwell there in great beauty and abundance. 

 Water still offers protection and support, and in it plants 

 and animals can secure a livelihood on easier terms than they 

 can on land. Many of them have lived in the water ever 

 since their ancestors started there; others, the aquatic seed 

 plants and insects, tried living on land but went back to the 

 water. Diverse histories lie behind the varied company of 

 plants and animals which now make up aquatic society, but 

 they have all become more or less adjusted to their common 

 experience of water life. 



Water environment. — Water is all around us. It covers 

 more than two-thirds of the earth's surface; ocean or streams 

 and lakes and little ponds are almost everywhere. Water 

 has more properties which are beneficial to living things than 

 has any other substance. It is seven hundred and seventy 

 times heavier than air, with a surface film upon which spring- 

 tails can jump, and water striders can walk. This surface 

 film is stronger than that of any other fluid except mercury-; 

 and although the water strider's feet bend it down into six 

 dimples, they do not break it. Snails and planarians glide 

 over its underside, and hydras hang head downward from it. 

 The little crustaceans, Scapholeberis (Fig. 124), cling to it, ven- 

 tral side up. On this side they are darkly colored, while their 

 backs are pale, their coloring like their position being just the 

 opposite of that in most animals. The larvae of mosquitoes 

 thrust their airtubes through the surface film and rest there, 

 tails up and heads down. Like the larvcc of the big diving 

 beetles (Fig. 209), and the pond scorpion, and many others, 

 they are buoyed up by their own air supply. 



The greater density of water gives it more supporting 

 power than air, so much so that the fairy shrimp (Fig, 123) 

 and a host of other animals can float motionless in it. All 

 types of animals and many plants have relied upon its sup- 



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