THE DISTRIBUTION OF ANIMALS 



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includes North America north of Mexico and also Greenland. These 

 two realms are mentioned together because they are very similar. The 

 mammals include bears, deer, beavers, squirrels, wild cats, and foxes. 

 The birds include finches, wrens, thrushes, ducks, and hawks which be- 

 long to the same or closely related species. The land bridge from 

 Alaska to Siberia probably accounts for the similarity of the animal life 

 in these two realms. 



Animal Distribution in the Ocean 



The sea is often called the cradle of life because all the phyla are 

 represented there and several, particularly the Echinodermata, are re- 

 stricted to marine habitats. The more primitive phyla are particularly 

 abundant in the sea, and our fossil records indicate that for a long time 

 all life was in the sea. There are three main types of marine habitats. 



Strand 



Pelagic zone 



Abyssal zone 



Fig. 29.3. The ocean zones. 



The Littoral Zone. This region is the shallow sea, usually extend- 

 ing out as an underwater shelf from some great land mass. In general, 

 its depth ranges up to about 600 feet. Light penetrates the water in 

 this zone and there is a good growth of marine plants especially in the 

 shallower portions of the littoral zone. Life is extremely abundant and 

 varied. Every available niche is occupied, and animals are even found 

 growing on one another. Because of the struggle for existence, every 

 organism must be very well adapted in order to survive. 



A very special area of this zone is known as the strand, which is 

 the area that lies between the low and high tides. Here the animals are 

 exposed to the drying effects of the sun's rays and the wind twice a day 

 when the sea recedes. Here, also, they must be able to resist the pound- 



