FRESHWATER FAUNA. 6i 



has far less lime than Globigerina ooze as it contains siliceous 

 radiolarians and numerous pteropod shells. The abysmal 

 region is peopled by a fair number of species scattered 

 throughout the same phyla as are found in the neritic zone. 

 Indeed there is every indication that the deep-sea has been 

 gradually peopled from the neritic region by immigrants. 



All the animals show more or less striking moditications. 

 All are carnivorous and many are phosphorescent. There 

 are numerous Crustacea which often attain enormous size. 

 Many of the large species are blind and of a light carmine 

 colour. The fishes, as at present known, are few in species, 

 all bony fishes or Teleostei^ and of extraordinary appearance. 



(2) Freshwater Fauna. — The freshwater fauna is very 

 diverse, as it includes dwellers in lakes, ponds, livers and 

 streams. It shows clear indications of having been derived 

 from the Neritic zone of the marine fauna, though doubtless 

 in some cases terrestrial animals have reverted to the 

 freshwater. 



The same divisions, into swimmers or Nekton, Plankton 

 and Benthos, can be made out, but the Plankton are very 

 few in number, including Protozoa and a few freshwater 

 Medusae. Amongst the swimmers we can notice two very 

 primitive orders of fishes, the Ganoidei and Dipnoi^ which 

 are confined to freshwater, apparently driven from the sea 

 by more specialised types. 



Two important points should be noted. Firstly, a great 

 number of freshwater forms can meet the physical vicissi- 

 tudes of their habitat by encapsufing themselves and 

 remaining dormant for some time (e.g,^ Atnceba^ Infusoria, 

 Rotifera, and Tardigrada). Secondly, the eggs and larvae are 

 hardly ever of the floating or free-swimming types, and are 

 commonly protected by a hard capsule. As the rivers have 

 been the lines of migration, the eggs and larvae would, if 

 floating, be borne back to the sea. 



The primitive freshwater types are especially interesting 

 as leading along the path towards the terrestrial fauna. 



2. Terrestrial Fauna. — The terrestrial fauna has evi- 

 dently been derived in the past from the aquatic. Only a few 

 phyla appear to have effected this migration. Of these the 



