THE OCEAN 345 



about but little, either lying on the bottom or burrowing into it. 

 Some, on the other hand, as many of those in the surface waters 

 (pelagic life), appear to be moving always. 



All the great groups of animal life are represented in the sea- 

 water. Even warm-blooded mammals (whales, seals, walruses, 

 etc.) abound in the frigid waters, among icebergs and ice-floes. 

 Some of these animals, like the seals and walruses, do not spend all 

 their time in the water, but frequently crawl up on the ice. From 

 this highest class of animals (mammals) down to the lowest, all 

 important groups are represented in the sea, though no birds spend 

 all their time in the water. The varieties of plant life are many, 

 but the forms we are most familiar with on land, are wanting. 



Not only are there many varieties of marine plants and animals, 

 but the largest living animals (whales) live in the sea. Many of the 

 sea plants, too, are of great size. Some seaweeds are six inches in 

 diameter, and some have a length greater than that of the tallest 

 trees. They are, however, not so bulky as large trees, and the 

 amount of solid matter which the largest seaweed contains is far 

 less than that of the largest tree. This would be seen if the large 

 seaweeds were allowed to dry. 



The life of the sea is important in many ways. Many of the 

 animals, such as fish, oysters, clams, crabs, lobsters, etc., are used 

 for food. The total value of food products derived from the sea 

 is probably not less than $500,000,000 per year. Other animals 

 furnish other articles of commerce. For example, the seal furnishes 

 fur and oil; the whale, oil and whalebone; the hide of the walrus 

 makes exceptionally strong leather. Coral and sponges, the pro- 

 ducts of animal life, are also articles of commerce. 



Many of the animals of the sea have shells or other hard parts. 

 These hard parts accumulate on the bottom of the sea when the 

 animals are through with them, and this is one source of the sedi- 

 ments of the sea bottom. If the shells, etc., get together in great 

 beds without much mud, sand, etc., they may in time be cemented 

 together, forming solid rock, called limestone. Most of the lime- 

 stone now found on land was formed in this way beneath the sea, 

 when the sea covered parts of the present continents. The animals 

 which make the heavier shells (or other secretions of calcium car- 



