THE YOUNG FISH 



known fish are given in Plates XXVIII and XXIX. In addition to the pig- 

 ment patterns so useful for identifying the individual species there are more 

 basic differences wliicli help to separate the various families. Of these the 

 most important are the shape of the gut, the position of the anus, the type of 

 fm arrangement, and the number of muscle segments and developing 

 vertebrae. This number is afi:"ccted, within limits, by the temperature at the 

 critical stage when the muscle segments and vertebrae are laid down. In 

 colder regions the number tends to be larger than in warmer, and indeed this 

 can be shown experimentally. Cold and warm treatment given to sea-trout 

 eggs of the same parentage given at the sensitive period resulted in differences 

 of as many as three vertebrae, sixty in cold and fifty-seven in warm treated 

 eggs, and experiments with herring show very similar results. 



Flat-fish start life bilaterally symmetrical just as the round fish, but as they 

 gradually metamorphose into the adult shape one eye starts to move over 

 the head (Plate XXX) and the fish then swims on its side. Nearly all the 

 common flat-fish have both eyes on the right side, but turbot, brill, megrim 

 and the topknots are left-sided. Every now and then, but remarkably infre- 

 quently, a left-sided plaice or flounder or a right-handed turbot is found. 



Special mention must be made here of the common freshwater eel of 

 which there are several kinds in the world. The European and the American 

 eels come from spawning grounds in the Atlantic and these two eels are very 

 alike, the main differences being in the number of vertebrae, 103-111 in the 

 American and 110-119 in the European. Although the European eels are 

 found from the Baltic and Iceland to the Mediterranean there is no evidence 

 at all of racial differences as there are with so many tish and other animals in 

 such a wide geographical range. The elucidation of their life history we owe 

 mainly to the Danish oceanographer, Dr. Johannes Schmidt. The eggs are 

 laid in deep water, 250 fathoms (500 metres) or more, in a fairly closely 

 confined area in the region of the Sargasso Sea, 20°-3o'^ N, 5o'^-6o' W, and 

 the eggs and the young larvae, at first only about j inch long, gradually rise. 

 Those of the European eel, hatched in the eastern part of the spawning area, 

 are drifted across the Atlantic by the Gulf Stream. Those of the American eel, 

 hatched in the western or south-western section are in a different part of the 

 current system and are carried towards the American coast. The larva, in its 

 oceanic form, is very transparent, thin and flattened like a willow leaf and 

 is called a 'leptocephalus' (Plate XXIX). This reaches about 3 inches in length, 

 swimming nearer and nearer the surface as it grows. On reaching the coastal 

 water the larvae gradually shrink in size and at the same time become rounded 

 instead of flat the glass eel stage; these swim with the flood tide but go to 

 the bottom at the ebb tide. They approach the river mouths and swimming 

 always against the current go up the rivers. The attraction of the fresh water, 



lOI 



