l6o A NATURAL HISTORY OF THE SEAS 



Eel (Anguilla rostrata) is situated in proximity to that 

 of the Old World species, the larval stages of both forms 

 having been taken in the same haul of the net. But 

 whereas the latter takes three years to cross the Atlantic, 

 the former's journey to its native ponds and rivers is 

 completed in a single year. How these minute larvae 

 can decide upon which direction to take, whether an easterly 

 or westerly one, is an unfathomable mystery. In the case 

 of the Eel the migratory routes are probably not determined 

 by custom as in the case of the Haddock, but by physio- 

 logical changes in the fish themselves, which demand 

 certain chemical conditions of the water. 



In other fishes the sudden or gradual change of food 

 supply and the irresistible forces exercised by the great 

 ocean currents, play their parts in determining the way 

 of a fish in the great waters. Only a few giants of the 

 race can be more or less independent of such factors and 

 shape their courses as their inclinations dictate. 



The expression " parasitism " is often loosely applied 

 to animals, and especially to fishes. The word is frequently 

 used to describe creatures which live together, whereas 

 the true parasite attaches itself to the host and lives solely 

 at his expense and often to his ultimate undoing. True 

 parasitic fish are not abundant, the best known are various 

 species of Lampreys and Hagfishes, some of which were 

 once recognised food supplies in this country. The 

 Lamprey is of primitive construction, having no true jaws 

 but a round suctorial mouth armed with numerous horny 

 teeth. By means of this structure, the creature attaches 

 itself to some larger fish and literally tunnels into the flesh, 

 at the same time drinking the blood until replete. In 



