Life of the Waters 363 



in water more than 3,000 feet deep. 2 Where the eggs are laid 

 is not known, but they hatch at the surface of the sea, into 

 ribbon-shaped, transparent larva* of about the thickness of 

 a visiting card. After about a year's time, during which 

 they are said to take no food, the larvae lose their ribbon 

 shape and assume the eel-like form. They now approach the 

 coasts and ascend the rivers in large numbers, forming what 

 are known as the "eel-fares" of late winter or early spring. 

 Like the Pacific salmon, the adults die after spawning, 

 though the manner of their demise is unknown. 



But the spawning and feeding migrations of marine animals 

 are not their only active movements. Many species seek the 

 deeper layers by day, coming to the surface at night. The 

 depth of this diurnal movement varies for different species, 

 but does not in general exceed 100 to 150 feet. In some 

 species, the wandering habit is restricted to the young, while 

 in others migrations are performed by the males only. Similar 

 diurnal movements occur in fresh water animals, as already 

 noted. 



There are also many species in which the young are found 

 at one level, while the adults occur at some other. This is 

 notably true of many bottom-living fishes, such as the cod 

 and halibut, whose eggs and young are found only at the 

 surface. Here it may be that differences in specific gravity 

 at the different ages explain these differences in distribution. 



A word here may not be amiss as to the tools of the 

 oceanographer. By what means has our knowledge of the 

 depths of the sea been obtained? In the study of oceanog- 

 raphy, as indeed in many other of the complex fields of 

 modern science, the biologist must be a physicist and chemist 

 as well. One of the most important implements of the 

 oceanographer is his sounding line. This is used not alone 

 for measuring depth but for carrying instruments of various 

 sorts. In the earlier deep sea expeditions, notably those of 

 the British ship "Challenger" (1872-76) rope lines were 

 used for sounding. On account of the heavy strain (the 

 weight of 20,000 feet of the line itself in water was nearly 

 250 pounds) a rope one inch in circumference was used. 

 The necessary length of this rope (over 30,000 feet) rendered 

 it awkward to handle and necessitated large space for its 

 accommodation. For trawling still heavier lines (up to three 

 inches in circumference) were required, which were still more 

 difficult to handle and to store. At the time of the "Chal- 

 lenger" expedition, Sir William Thompson (Lord Kelvin) 



2 Recently the Danish oceanographer, Johannes Schmidt, has appar- 

 ently discovered the spawning grounds of both European and American 

 eels southwest of the Bermuda islands. 



