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 adyssa/ fauna, a “ttoral fauna, and so on. Let us. 
briefly consider this grouping of animals according to their 
haunts. 
Pelagic.—The pelagic fauna includes all the animals of 
the open sea, both drifters (Plankton) and swimmers. 
(Wekion). 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 
Algze 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 Voctiluca 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, Meduse and Medusoids, 
Siphonophora and Ctenophora, many “‘ worms,” a few Holo- 
thurians, a legion of Crustaceans, a few Insects (Halo- 
batide), such Molluscs as Pteropods, Heteropods, and 
many of the Cephalopods, such Tunicates as Sa/pa and 
Pyrosoma, many fishes, a few turtles and snakes, besides 
some well-known birds and mammals. There are also 
hosts of /avva/ forms which are pelagic for a time. 
The fauna of the open sea is representative, but there are 
