158 FORMATION OF CORAL REEFS. 



while others would be killed at once by the same 

 pressure ; and the latter naturally seek the shal- 

 low waters. Every fisherman knows that he 

 must throw a long line for a Halibut, while with 

 a common fishing-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 florae as well as the 

 faunas of the ocean have their precise boun- 

 daries. 



This wider or narrower range of marine ani- 

 mals 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 hav- 

 ing a wide range of distribution in ocean-depths, 

 have a special apparatus of water-pores, so that 

 the surrounding element penetrates their struc- 

 ture, 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 variation? of 

 climate and of atmospheric pressure. 



Of all air-breathing animals, none exhibits a 

 more surprising power of adapting itself to great 



