I. Introduction.' 



I Tn the »First Report on Eel Investigations«, vol. XVIII des Rapports et Procès- 

 i. Verbaux du Conseil International pour l'Exploration de la Mer«, published on 

 the 25th of November 1913, I have set forth the results which I considered as 

 having been obtained by my work in connection with classification of the European, 

 American and Japanese fresh-water eels, at the same time making mention of the 

 fact that the characterization of the species necessitated the examination of a great 

 number of specimens. As regards the characters to be considered in distinguishing 

 the species, these were divided into two classes; a) permanent characters, 

 comprising those which are present in their ultimate form at a very early stage 

 in the life of the fish, and b) variable characters, being those which in the 

 course of growth are subject to greater or lesser alteration. Owing to the very 

 considerable labour involved by investigation of hundreds or thousands of specimens, 

 I had not been able to thoroughly examine more than a few of the characters in 



•question. These were, 1) The number of vertebrae, a permanent character, 

 and 2) the ano-dorsal distance, or distance between the initial points of the 

 dorsal and anal fins, expressed as a percentage of the total length, which is a 

 variable character. Detailed investigation of these two characters showed that both 

 were of systematic importance. Countings of vertebrae enabled us to prove, that 

 the European, American and Japanese eels belong to three different species. Only 

 in the case of the first-named was sufficient material available to permit of more 

 thorough investigation, and discussion of the important question as to whether or 

 no several »smaller species« or »races« could be shown to exist within this 

 species. With regard to this, I arrived at the following result: 



Countings of vertebrae of numerous specimens from different localities 

 within the area of occurrence of the European eel seem to indicate that we have 

 everywhere to deal with one and the same species. This is suggested, partly 

 by the high degree of uniformity between the different samples, the averages of 

 which exhibit but slight variation, and partly by the fact that the slight variation 



• The first part of the present work has already been published in a paper entitled »On the 

 classification of the Fresh-water Eels {Anguilla)« in Meddelelser fra Kommissionen for Havundersogelscr, 

 Serie Fiskeri, Bind IV Nr. 7, issued on November 30th 1914. 



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