CHAPTER XXVII 



GEOGRAPHICAL DISTRIBUTION OF ANIMALS 



As similar animals tend to occur where the conditions of 

 life are similar, we are warranted in speaking of a pelagic 

 fauna, an abyssal fauna, a littoral fauna, and so on. Let us 

 briefly consider this grouping of animals according to their 



haunts. 



Pelagic. — The pelagic fauna includes all the annuals ot 

 the open sea, both drifters {Plankton) and swimmers 

 (Nekton). The physical conditions in which they live are 

 very favourable— there is room for all, sunshine without 

 risk of drought, and an evener life throughout the day and 

 throughout the year than is to be found elsewhere except m 

 the abysses of the deep sea. Moreover, the minute pelagic 

 Alg^ afford an inexhaustible food-supply to the animals. It 

 is not surprising, therefore, to find that the open sea has 

 been peopled from the earliest times of which the rocks 

 give us any life record. 



The fauna is representative, exhibiting great variety of 

 types, from the minute Noctiluca which sets the waves 

 aflame in the short summer darkness, to the giants of 

 modern times— the whales. It includes a few genera of 

 Foraminifera, rich in species, most Radiolarians, Dino- 

 flagellata, many Infusorians, Medusae and Medusoids, 

 Siphonophora and Ctenophora, many " worms," a few 

 Holothurians, a legion of Crustaceans, a few Insects (Halo- 

 batid^e), such Molluscs as Pteropods, Heteropods, and 

 many of the Cephalopods, such Tunicates as Salpa and 

 Pyrosoma, many fishes, a few turtles and snakes, besides 

 some well-known birds and mammals. There are also 

 hosts of larval forms which are pelagic for a time. 



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