2 UNDERWATER GUIDE TO MARINE LIFE 



animal. His size has made it extremely difficult for him to project himself intc 

 the worlds of the small living things that swarm about him and has restrictec 

 his vision to include mostly the forms of life that are, like him, giant. 



fiis terrestrial point of view has caused him to make a fundamental mistake 

 in his conception and naming of the planet on which he lives, for this planet is 

 not one characterized by dry land nearly as much as by the sea. No other 

 planet, as far as is known, has a sea. Had man been aware of this from earliest 

 time, he might well have named this planet "Sea" rather than "Earth." 



The sea is not only the most unique characteristic of earth, but it clearly 

 dominates the land in surface area covered. About 70 per cent of the earth's 

 surface is covered by the sea, an area encompassing about 139 million square 

 miles. By contrast, all of the lakes and rivers add up to only about one million 

 square miles. The sea averages about two and one-half miles in depth, whereas 

 the land rises an average of only slightly more than four-tenths of a mile above 

 sea level. The volume of sea water is about 334 million cubic miles, a volume 

 so great that it has been estimated that if all the earth were made level, the 

 sea would everywhere cover it to a depth of one and one-half miles! 



PHYSICAL FACTORS OF THE SEA 



Water 



The remarkable compound to which all life is responsible and which gives 

 the sea most of its important characteristics is water. 



First, water is most common on earth in its liquid state, and it is fortunate 

 that the temperature range of most of the earth lies between water's freezing 

 point and boiling point, that is, between 32° and 212° Fahrenheit (salt water 

 freezes at about 29°). It is not merely happenstance that the temperature limits 

 of life lie between the temperature limits of liquid water. 



Second, water is one of the few compounds that expands, becoming less 

 dense, when it changes from the liquid to the solid state. This enables ice to 

 float, a fact of the greatest significance, for if ice sank, the depths of the sea, 

 and eventually the whole of polar seas, would freeze solid. 



Third, water is the most universal of all solvents, able to dissolve large amounts 

 of a wide variety of substances that are absolutely necessary for life. In fact, 

 protoplasm is 50 per cent (flour beetles) to 98 per cent (jellyfish) water. 

 This liquid virtually bathes every fiber of living bodies, carrying food and 

 oxygen to cells and carting off metabolic wastes such as carbon dioxide and urea. 



Fourth, water has a very high capacity to store heat. The great body of 

 ocean water is the storehouse of the earth's heat. Without the oceans, the dark 

 side of our planet would come close to freezing nightly. 



Fifth, water is dense and gives support and buovancv to the animals that 

 live in it. This has the important consequence of solving the problem of support 

 for marine animals. Strong legs, arms, and skeletons, necessary for support on 

 land, are lacking in the sea. Instead, the bodies of marine animals are modified 

 for locomotion and for offense or defense, and not usually for support. Plants 

 in the sea have no strong supporting stem or trunk. Because of water's buoyancy. 



