270 FISHES 



In the littoral regions there are various kinds of habitat to 

 which different species are adapted ; there are fishes which live 

 among the weeds which are attached to the rocks, such as the 

 wrasses or Labrida; ; others which burrow in the sand like the 

 sand-eels (Ammodytidaj) and weevers (Trachinidae), or lie up- 

 on the bottom like the flat-fishes (Pleuronectidae), the anglers 

 (Lophiidas), and the skates and rays (Batoidei) ; others which 

 feed mostly on the bottom like the cod family (Gadidae), others 

 which live in shoals and feed in open water, either like the her- 

 rings (Clupeidao) by the filtration method, or like the mackerel 

 and the picked dog-fish {Acajithias), by preying on smaller 

 fishes. These different habitats are not peculiar to any geo- 

 graphical region but occur in all parts of the world with similar 

 fishes associated with them. Geographical restrictions of 

 families or genera are determined for littoral fishes chiefly by 

 temperature ; there is nothing to prevent the same species of 

 littoral fish extending round all the coasts of the world, but in 

 fact we find each zone of latitude characterised by its own fish- 

 fauna. There are four great continental lines of coast running 

 generally north and south, and along these there are four parallel 

 series of littoral fishes ; in each of these, although species may 

 be and usually are different, there is a general similarity between 

 the fish population of different coasts in the same latitude. 

 Thus we can distinguish Arctic fishes, those of the temperate 

 zones, those of the tropical zone, and Antarctic fishes. There 

 is even a certain degree of similarity between the fishes of the 

 north temperate and south temperate zones, but less resemblance 

 between the Arctic and the Antarctic. In the surface waters of 

 the great oceans there is not so much difference between the 

 temperature of different zones and no difference of habitat ; 

 but even here there are differences of habits. Some pelagic 

 fishes are large and powerful swimmers like the sharks and 

 sword-fishes, others drift passively in the water like the sun- 

 fishes, others even make their homes in the floating weeds like 

 Pterophryne in the Sargasso. In the abysses the conditions of 

 low temperature and great pressure are similar all over the 

 world and sunlight is everywhere wanting ; but even here there 

 are fishes which live on the bottom and others which swim 

 actively in the water. 



The most characteristic tropical fishes, especially in the 



