30 THE TUNDRA AND ITS FAUNA 
gives him almost nothing. It is a commonplace of 
physiology that in cold climates his need of fat is great, 
but in this respect he resembles all the other animals 
of the region, all of whom seek fatty food, and store 
up in their bodies reserves of this heat-producing sub- 
stance. The food from which the larger marine animals 
obtain their supplies of fat is almost always, directly or 
indirectly found in the myriads of small crustacea which 
haunt polar seas, and whose bodies contain much oil. 
Arctic seas are indeed specially characterized by their 
wealth of small crustacea and of marine mammals, and 
the two facts are closely connected. The crustacea feed 
the fish of the polar seas, which, though of relatively 
few species, are often extraordinarily abundant as 
individuals. The seals and toothed whales feed upon 
fish. Small crustacea are also devoured by molluscs, 
of which the mud-haunting forms are devoured by the 
walrus, and the floating forms by the whalebone whales, 
so that the crustacea form the basis of the marine food- 
supply. They are in their turn fed by the more minute 
forms of life, especially by the minute plants, such as 
the diatoms, so abundant in polar seas. 
The few survivors of the ill-fated Greely expedition 
kept themselves alive by the collection of ‘ shrimps’ 
in the shore-water. These ‘ shrimps’ must have been 
largely the small crustaceans called amphipods, which 
are enormously abundant in polar seas, and the fact 
that they were so utilized may be taken as emblematic 
of Arctic conditions. 
Apart from crustacea, the littoral fauna of the Arctic 
is fairly rich. A considerable number of large molluscs 
occur, including in the ‘ gapers’ Mya and Saxicava 
mussel-like forms, which constitute an important part 
of the food of the walrus. Where molluscs abound, 
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