198 



ANIMALS OF LAND AND SEA 



perate and cold, and uniform and changing regions; dry, 

 damp and wet areas, permanent or variable. All these features, 

 together with the chemical variability of the soil, are reflected 

 in the flora of the land, and all these features, plus the super- 

 added features of the flora, affect the animals. In consequence 

 land animals have become subdivided into an indefinite number 



of dift'erent forms or minor types each most 

 efficient within a small range of conditions. 



But all land animals have one thing in 

 common; they must seek their food; it will 

 not come to them. Therefore land animals 

 are almost wholly of those types, arthropods 

 and vertebrates, best fitted for locomotion, 

 with representatives of some other types of 

 fair locomotor powers. 



In the sea conditions are quite different. 

 The temperature range is small. At no place 

 temperatures of less than 28.4° are to be 

 found, while high temperatures, even in the 

 tropics, are confined to a thin superficial layer. 

 The great bulk of sea water ranges in tem- 

 perature between 35° and 60°. The chemical conditions are only 

 slightly variable. The salinity varies somewhat, but the differ- 

 ent salts are everywhere present in about the same proportion. 

 Motion affects only the surface waters, and is negligible except 

 along the shores. 



In the sea food substances float everywhere suspended in 

 the water, drifting back and forth and up and down. While 

 useful, powers of locomotion are not necessary for the creatures 

 in the sea; if they cannot seek their food it will be brought to 

 them. 



Thus in the sea the food relations of the animals are of three 

 kinds; some go after it, as do the animals on land; some attach 

 themselves or burrow in the bottom and let the water do the 

 work of bringing food to them; and some float suspended in 

 their food supply. 



Figs. 584-585. 

 Discodrilid worms. 



For explanations of 

 the figures see p. xxx 



