DISTRIBUTION AND MIGRATIONS 265 



{Ostariophysi) has evolved entirely in fresh water, only a few of 

 its members being secondarily marine. 



Fresh-water fishes may be conveniently divided into two 

 main categories: (i) those spending part of their life in the sea; 

 (2) those living permanently in fresh water. Among the 

 members of the first group are the Grey Mullets (Mugilidae) , 

 which inhabit the estuaries and may penetrate for considerable 

 distances up the rivers, as well as fishes like the Flounder 

 (Flesiis) and Stickleback {Gasterosteus) , equally at home in 

 salt or fresh water. The Three-spined Stickleback (G. aculeatus) 

 has a very wide range, being found on the coasts and in the 

 rivers of the arctic and temperate regions of the Northern 

 Hemisphere, extending as far north as Greenland, Alaska, and 

 Kamchatka, and as far south as Japan, California, New Jersey 

 and Spain. In northern regions it is essentially a marine fish; 

 in the British Isles it is equally common on the coasts and in 

 the rivers; and in Spain and Italy it is almost entirely confined 

 to fresh water. 



Also pertaining to the first category are the catadromous 

 fishes, forms which feed and grow in fresh water, but return 

 to the sea to breed. The Common Eel {Anguilla anguilla) is the 

 best known of these fishes, and its distribution in Europe is of 

 special interest when considered in relation to its life history, 

 and particularly to its breeding habits. These will be described 

 in a later chapter {cf. p. 288), but it may be noted here that 

 the adult Eels cross the Atlantic to spawn in deep water to 

 the south of Bermuda, the larvae subsequently making their 

 way slowly in an easterly direction, reaching the coasts of 

 Europe when about two and a half years of age. Here they 

 become transformed into elvers, which are about three years 

 old when they enter the rivers of the British Isles. A glance at 

 the map (Fig. 106) shows that the distribution of the species 

 in fresh water is a wide one, extending from Iceland and northern 

 Norway to Morocco, and throughout the countries bordering 

 the Mediterranean. Now these are just the coasts that the 

 larvae reach at a time when they are ready to become elvers, 

 or the further regions to which the elvers or young eels are 

 subsequently able to make their way. 



Among other catadromous forms mention may be made of a 

 tribe of small fishes allied to our own Salmon and Trout that 

 are found in the Southern Hemisphere. These Galaxiids 

 {Galaxiidae) are for the most part confined to the ri\ers of the 

 southern extremity of South America, the Falkland Islands, 



