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 along 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 flore as well as the 
faune 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 variations of 
climate and of atmospheric pressure. 
Of all air-breathing animals, none exhibits a 
more surprising power of adapting itself to great 
