ANIMAL LIFE 169 



In depths beyond 300 fathoms the animals 

 are characterised by reddish and dark colours : 

 thus the medusae show dark brown and red 

 colouis, the pteropods dark violet, the fishes 

 black or dark violet, the worms and crusta- 

 ceans red. 



These variations of colour seem to be corre- 

 lated with the amount of sunlight penetrating 

 to the various depths, for the dark-coloured 

 bathypelagic forms are found in deeper water 

 in the tropics than in temperate regions where 

 the penetration of light is less. It has been 

 shown by the " Michael Sars " Expedition 

 that the dark-coloured fishes undertake verti- 

 cal migrations during the night, and have 

 consequently a very considerable adaptation 

 to changes of temperature and pressure. The 

 amount of pigmentation in certain forms like 

 medusae and crustaceans seems also to be 

 regulated by the amount of light, increasing 

 in intensity with increase of depth. 



Phosphorescent Light. — We have already 

 referred to the extraordinary fact that no 

 phosphorescent organisms have been found 

 in fresh water, even phosphorescent bacteria 

 requiring sodium chloride in the medium in 

 which they are cultivated. The power ©f 

 emitting phosphorescent light is widely dis- 

 tributed in nearly all groups of marine organ- 

 isms, including bacteria, peridineans, flagel- 



