ANIMAL LIFE 167 



that met with at the lower limit of the photic 

 zone, where the temperature is 40° F. It 

 follows that organisms sink twice as fast in 

 the warmer water above as in the colder 

 water below, and it is necessary for them 

 to develop various kinds of floating apparatus, 

 so as to increase or diminish their surface 

 resistance. These floating devices include 

 secretion of fats and oils, development of air- 

 bladders, reduction of size, variation in shape, 

 and production of various forms of append- 

 ages. As in the case of cold-water marine 

 plants, polar marine animals do not require 

 to develop suspension-organs, because of the 

 small variation in the viscosity of the water 

 in surface and deep layers. 



Relation of the Pelagic Fauna to Penetration 

 of Light. — The animals captured near the 

 lower limit of light-penetration, that is in 

 tropical regions in depths of about 500 fathoms, 

 have been called bathypelagic or twilight 

 animals. There is at this depth a rather 

 sudden change of temperature, salinity and 

 viscosity, and in consequence we find many 

 adaptations to these peculiar physical condi- 

 tions, for instance as regards, colour and 

 amount of pigmentation, and the develop- 

 ment of floating contrivances, and of eyes and 

 light-organs. 



Floating at the surface of the ocean we have 



