BONY FISHES 



285 



that the fish exhibits many deviations in, its body structure (as, indeed, 

 is the case in all widely - distributed animals, e.g., lions, domestic 

 animals). Naturalists accordingly distinguish races or varieties of 

 herrings, comprised within two groups — viz., deep-sea and littoral 

 herrings. The former inhabit the high seas, the latter (to which 

 belongs the herring of the Baltic) the shallow waters of the coast. The 

 herring is generally met with in the free open water not far from the 

 surface. 



C. Colour. 



Hence also its colour is bluish-green, like that of the sea, whilst its 

 scales display the silvery glitter of the sunlit waves. (Protective 

 colouring. Compare, on the other hand, the colour of bottom fish and 

 of predaceous fish which lie in hiding among water-plants, e.g., pike and 

 perch.) 



D. Food. 



In the open water the herring finds its food, which consists chiefly of 

 all kinds of small free-swimming animals (the so-called Plankton, or 

 floating fauna). It feeds more especially on minute crustaceans belonging 



Copepod (Diaptomus). A Fkesh-Watbr Species. 

 Female with egg-bags attached to the terminal abdominal segment. (Magnified about 



thirty diameters.) 



to the Order Copepoda, or water-fleas, creatures not larger than from one 

 to a few millimetres, but occurring frequently in such enormous quantities 

 as to colour the water red for the space of many square miles, and 



19 — 2 



