ANIMAL LIFE 159 



the metabolism in animals, and this fact may 

 explain many puzzling problems in marine 

 biology. 



We shall in the first instance refer to animals 

 which inhabit the surface, sub-surface and 

 intermediate waters of the ocean (Plankton), 

 and then to those which are attached to or 

 crawl over the ocean-floor (Benthos). 



Plankton Fauna. — The great variety of 

 plant-life in the surface and sub- surface waters 

 of the ocean, and the variations brought about 

 by changes in temperature, viscosity and 

 other physical conditions, have been referred 

 to at the beginning of the preceding chapter. 

 Animal life is equally abundant, and in even 

 greater variety, in the surface and sub-surface 

 waters, and is grouped into (1) plankton, 

 both neritic and oceanic — animals carried 

 along by the currents — and (2) nekton — 

 animals which can swim against currents. 



Almost all classes of marine animals are 

 represented in the plankton fauna in either 

 the colder or the warmer areas : reptiles, 

 fishes, tunicates, crustaceans, molluscs, worms, 

 coelenterates and protozoa. The sponges are 

 unrepresented. The great class of echino- 

 derms seems to be represented only by the 

 pelagic holothurian Pelagoihuria, although 

 the larval stages are numerous, especially 

 where the sea is shallow, and the large class 



