PLANT LIFE 137 



and of the floor of the ocean, presents great 

 uniformity of physical conditions when com- 

 pared with those prevaihng in the neritic 

 area. Temperature, salinity and viscosity 

 vary with latitude, but the pelagic types of 

 organisms are more widely distributed in the 

 open ocean than in the neritic area. 



Throughout the whole photic zone chloro- 

 phyllous plants (mostly algae) are abundant 

 and universal, while the animals of the zone 

 are both herbivorous and carnivorous. 



II. — The Aphotic or Deep-Sea Zone extends 

 from the lower limit of the photic zone down 

 to the bottom of the greatest " deeps." Day- 

 light does not penetrate to the waters of this 

 zone, the sun's rays being almost wholly 

 absorbed in passing through the upper layers, 

 but phosphorescent light, produced by organ- 

 isms, appears to play a great role throughout 

 the whole zone. The temperature is low 

 except in some enclosed seas. There is little 

 variation in the salinity and viscosity. The 

 pressure varies with the depth, but physio- 

 logical functions are little disturbed even by 

 excessive pressure, when once equilibrium is 

 established within and without the organism. 

 Living chlorophyllous plants (algae) are 

 absent, and the animals are carnivorous and 

 scavengers or mud- eaters. The bottom con- 

 ditions on the floor of this deep-sea zone are 



