SALMON).
When a migratory fish comes inshore or ascends the rivers to spawn, the term anadromous is used, with its counterpart cata dromous, when the spawning occurs in deepish salt water; other useful terms for different types of movement have been defined by Meek. The researches of Schmidt show that the chief breeding area of the European eel, Anguilla anguilla, is in deepish water in the south-western part of the north Atlantic, somewhat north of the West Indies. Thence the transparent larvae (leptocephali) gradually make their way, helped by currents, towards the European coasts. As they approach, being over two years and a half old, and having traversed, it may be, 2,000 miles, they undergo a remarkable metamorphosis, from a knife-blade-like to a slender cylindrical shape, and are then known as elvers. These ascend the rivers and often pass to lakes, feeding voraciously and growing quickly, continuing their nutritive life for 5-8 years, the females taking longer to assume the characters of maturity. The physiological restlessness that then sets in prompts a migration down the rivers and out to sea, a journey taking several months. Adult eels seem to die after spawning, so that there is no return journey to fresh water except for the next generation (see EEL).
Similarly the large marine lamprey, Petromyzon marinas, as cends the rivers to spawn and succumbs soon afterwards. The
larvae remain larvae, known popularly as "niners," technically as "Ammocoetes," for three years or so, and then, assuming the adult characters, they go down to the sea to put on flesh. The basal fact in the interpretation must be that lampreys were originally freshwater animals, as most of the lamprey species always are, whereas the common eel had originally a deep-water marine home, and took secondarily to fresh waters. In the interior of New York State Petromyzon marinas does not go down to the sea, but passes from river to lake, and from lake to river. Such secondary tele scopings of the typical life-history are very significant.