458 THE WONDER OF LIFE 



minute transparent free-swimming larva, suggestive of a 

 tadpole. For some hours it enjoys a free-swimming life, 

 propelling itseM by means of its tail. At this stage it has a 

 brain and a dehcate dorsal nerve-cord, a supporting dorsal 

 axis (or notochord) in its tail, a brain-eye, a ventral tubular 

 heart, and two or more pharyngeal gill-sHts — ^aU of them 

 distinctively vertebrate characters. But it does not fulfil 

 the promise of its youth ! It soon gives up its active life, 

 fastens itseK by its head to seaweed or stone, and almost 

 immediately falls victim to rapid degeneration. The 

 nerve-cord is lost and the brain-eye ; the tail shrinks and 

 disappears, devoured by its own phagocytes ; the posterior 

 part of the body becomes twisted dorsally through 180° — 

 and within a few hours the creature begins to look hke a 

 miniature Ascidian — one of the most signal instances of 

 individual degeneration in the whole animal kingdom. 



Eels. — ^There is a fascination in the Ufe-history of 

 the freshwater eel, though the mystery has been in part 

 removed. From inland ponds and quiet stretches of rivers 

 the full-grown eels migrate on autumn nights seawards ; 

 they pass out to sea into deep water, and probably die after 

 reproduction, for they never return. Obscurity stiU hangs 

 over the deposition and fertihzation of the eggs and over 

 the early stages of development. The transparent Lepto- 

 cephalus larvse are found near the surface, and are for a 

 year or more pelagic. From the open sea, the young eels, 

 when they have become cyhndrical in shape, migrate 

 shorewards and pass up the streams in a marvellous 

 procession or eel-fare. 



On the Michael Sars (1910) expedition, the larvae of the 

 common eel were found not only on the Continental slopes, 

 but also in mid-ocean over the greatest depths, both over 



