i] THE CATEGORIES OF LIFE 19 



to migrate over wide areas of sea. These are all 

 grouped together to form the category of life known 

 as the Nekton. Comparatively few marine animals 

 in addition to the fishes are nektic in habit. The 

 whales, seals, dolphins and porpoises are the next 

 largest group, and in addition to these and the fishes 

 only a few invertebrates, such as the squids, the 

 cuttle-fishes, and the giant calamaries are actively 

 moving animals; and perhaps some pelagic worms 

 and medusae may also so be described. According 

 to their habits the fishes may be further subdivided 

 into demersal and pelagic species. The demersal 

 fishes are those which live on the sea bottom and obtain 

 their food from animals which also live there. Most 

 of the commoner fishes belong to this category ; the 

 sole, plaice, whiting, haddock, cod, skate and ray, 

 turbot and brill and many others are examples. The 

 pelagic forms are the minority, and the best-known 

 examples are the herring, mackerel, sprat, pilchard, 

 and probably also the salmon while it inhabits the sea. 

 As a rule the distinction between demersal and 

 pelagic fishes is a very real one, and it is exceptional 

 for a fish which lives at the sea bottom to approach 

 the surface ; if it does the habit may be regarded as 

 an abnormal one. If we trawl demersal fish such as 

 the cod or whiting from even the moderate depth of 

 20 fathoms, and take care that the animals are not 

 injured by the action of the fishing gear, we see 



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