CHAPTER XXVIII. 



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. 



The Pelagic Fauna. 



The pelagic fauna includes all the animals of 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 in the abysses of the 

 deep sea. Moreover, the minute pelagic Algae afford an 

 inexhaustible food supply to the animals. It is not sur- 

 prising, 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, all the Radiolarians, many Infusorians, Medusae 

 and Medusoids, Siphonophora and Ctenophora, many 

 " worm " types and a Holothurian, a legion of Crustaceans 

 and a few Insects (Halobatidae), 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. 



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