WATER 



295 



get oxygen from the air or, as in other larvae, by gills from the water. 

 Animals living in water are often shaped for living under stones. 

 They are frequently protected from their enemies by having the 

 same color or appearance as the bottom of the sea. They always 

 have devices for catching their food, as evident in the mouth parts 

 of the crayfish, or cilia in unicellular animals which sweep food in 

 a water current. The tiny microscopic animals and plants, called 

 collectively the plankton, which serve as food for other animals, are 



Wright Pierce 



On the left side of the illustration, we see only the type of vegetation that is charac- 

 teristic of the desert, while on the right, where the land has been well irrigated, a grove of 

 orange trees produces an abundant yield of fruit. 



found only in the upper levels of the water, because light penetrates 

 only a few feet and the oxygen supply is deficient at greater depths. 

 Sunlight heats the water rather uniformly to a depth of 30 to 

 50 feet in small bodies of water and to a greater depth in the ocean, 

 due to the stirring up of the surface water by wind. Great depths 

 have very low temperatures. Life there is naturally much re- 

 stricted and few living things are found. There are some fishes 

 living at great ocean depths which are adapted to withstand the 

 great pressure of the tons of water pressing in upon them. How- 

 ever, we know very little about their internal structure because they 



