FLORIDA REEFS. 45 
We all live iiiKlor tlie 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, Avhich 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 long line for a halibut, while with a common fishing- 
rod he Avill catch plenty of perch from the rocks near the shore ; .and the 
differently colored bands of sea-weed revealed b^^ 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 florte 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, altliough 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 
