LIFE HISTORIES 357 



centric rings of small calcareous plates. These plates vary in 

 size and form successive zones, the outer rings of each zone being 

 narrower and more closely crowded together while the inner 

 are broader and therefore farther apart. The inner rings are 

 formed in spring when the eel begins to feed after the period of 

 hibernation, while the narrower rings correspond to the diminu- 

 tion in the rate of growth which takes place in autumn. Each 

 zone therefore indicates one year of growth and the total 

 number of zones shows the number of years which has gone to 

 the growth of the scale. As however the scales are not 

 developed until the eel is two years of age, this number has to 

 be added to the number of annual zones in the scales to obtain 

 the total age of the fish. In this way it was found that male 

 silver eels, that is males which had assumed the breeding livery 

 and were descending to the sea, were for the most part 5^ to 

 6\ years old, while the females were two years older or j\ to 

 8^ years. In the eel, therefore, as in other fishes the males 

 reach maturity at an earlier age than the females. A few 

 males were found which were only 4% years old and a few 

 which were J\, but only in one case was the maximum of 8^ 

 years indicated. Females more than 8^ were also rare, but the 

 following record of older females is given with the corresponding 

 lengths : — 



86 cm. (34 '4 in.) ....... io\ years. 



87 cm. (34*8 in.) 11^ years. 



96 cm. (38-4 in.) i\\ years. 



g6 cm. (38*4 in.) . . . . . . . \\\ years. 



96 cm. (38*4 in.) \i\ years. 



Apparently in individual cases eels live to much greater ages 

 than these without assuming the breeding livery or migrating 

 to the sea. It is supposed that for some unknown reason such 

 eels are sterile ; it is certain that they show no signs of matura- 

 tion of the reproductive organs. One specimen which died in 

 an aquarium in 1877 was stated to have lived in captivity for 

 twenty-two years, and another is mentioned by Day which lived 

 for upwards of thirty-one years in a well and was then choked 

 by a frog which it tried to swallow. 



It is not certain how long a time elapses between the 

 descent of the silver eels to the sea and the actual process of 

 spawning. The descent takes place in autumn and the evidence 



