FLORIDA REEFS. 45 



We all live under the pressure of the atmosphere. Now, thirty-two 

 feet under the sea doubles that pressure. At the depth of thirty-two feet, 

 then, any marine animal is under the pressure of two atmospheres, — that 

 of the air, which surrounds our globe, and of a weight of water equal to it; 

 at sixty- four feet he is under the pressure of three atmospheres, and so on. 

 There is a great difference in the sensitiveness of animals to this pressure. 

 Some fishes live at a great depth, and find the weight of water genial 

 to them ; while others would be killed at once by the same pressure ; 

 and the latter naturally seek the shallow waters. Every fisherman knows 

 that he must throw a lony; line for a halibut, while with a common fishino;- 

 rod he will catch plenty of perch from the rocks near the shore ; and the 

 differently colored bands of sea-weed revealed by low tides, from the green 

 line of the ulvas through the brown zone of the common fucus, to the rosy 

 and purple-hued sea-weeds of the deeper water, show that the florse as well 

 as the faunae of the ocean have their precise boundaries. 



This wider or narrower range of marine animals is in direct relation to 

 their structure, which enables them to bear a greater or less pressure of 

 water. All fishes, and, indeed, all animals having a wide range of distribu- 

 tion in ocean-depths, have a special apparatus of water-pores, so that the 

 surrounding element penetrates their structure, thus equalizing the pressure 

 of the weight, which is diminished from without in proportion to the 

 quantity of water they can admit into their bodies. Marine animals differ 

 in their ability to sustain this pressure, just as land animals differ in their 

 power of enduring great variations of climate and of atmospheric pressure. 



Of all air-breathing animals, none exhibits a more surprising power of 

 adapting itself to great and rapid changes of external influences than the 

 condor. It may be seen feeding on the sea-shore under a burning tropical 

 sun, and then, rising from its repast, it floats up among the highest summits 

 of the Andes, and is lost to sight beyond them, miles above the line of 

 perpetual snow, where the temperature must be lower than that of the 

 Arctics. But even the condor, sweeping at one flight from tropic heat to 

 arctic cold, although it passes through greater changes of temperature, does 

 not undergo such changes of pressure as a fish that rises from a depth of 

 sixty-four feet to the surface of the sea ; for the former remains within the 

 air that surrounds our globe, and therefore the increase or diminution of 

 pressure to which it is subjected must be confined within the limits of one 

 atmosphere ; while the latter, at a depth of sixty-four feet, is under a 



