150 THE BOOK OF NATURE STUDY 



open sea life (when they were flat from side to side, transparent 

 "glass-eels" about 7 inches long); they become less delicate, 

 shorter, cylindrical, and coloured ; they swarm up the rivers by 

 day and rest at sunset ; they ascend to slow-flowing reaches and 



FlG. 54. A small specimen of the common eel Anguilla vulgaris. There 

 is a long median fin along the back and along the ventral surface. 

 There are no pelvic fins. The gill-openings are very small. 



to ponds ; they grow large slowly, but never breed in fresh 

 waters ; they return fully grown, five or six years old, to the sea 

 to the deep waters, and probably die after giving rise to a new 

 generation. 



REFERENCES. Bashford Dean, Fishes, Living and Fossil (Macmillan & Co., 

 London, 1895); Pettigrew, Animal Locomotion (International Scientific Series, Kegan 

 Paul & Co., London, 1874); T. W. Bridge, Cambridge Natural History, vol. vii. 

 (Macmillan & Co., London, 1904); A. Gunther, Study of Fishes (A. & C. Black, 

 Edinburgh, 1880). 



