- 171 - 



reduction in the volume of the body which the metamorphosis brings about. The form 

 of the body now approximates to the cylindrical, the body height being reduced and the 

 greater part of the dorsal and lateral aspects of the body becoming covered with pig- 

 ment. The distance from the beginning of the anal fin to the end of the tail is almost 

 as in the previous stage. 



It is in the nature of the case, that there cannot be any sharp separation between 

 the 6 stages mentioned, of which the 2nd— the 4th seem to be of the shortest duration. 

 I think it may be of importance, however, for future investigations of the distribution of 

 the eel larvae and their migration to the coasts from deep water, to characterise briefly 

 the various stages of metamorphosis in which they are found, and I believe therefore that 

 such a grouping of the developmental stages will prove useful, and not the least when 

 it is a question of determining at what period of the year the various stages of develop- 

 ment occur. 



As was already known from Grassi and Calandruccio's investigations, a reduction 

 occurs in the total length after metamorphosis. In order to determine the amount of this 

 reduction we may either keep the fish in an aquarium and measure them during meta- 

 morphosis or we may possess so large a material that the measurements can be repre- 

 sented graphically and thus show the amount of the reduction. It has not been known 

 how large this reduction is, which occurs during the development from the Leptocephalus 

 stage and onwards, as Grassi and Calandruccio do not state the average length of their 

 Leptocephali (they only say that the lengths varied from 60—77 mm., 1. c. p. 9). It is 

 therefore of some interest to ascertain this point from the material brought from the 

 Atlantic by the "Thor". Whilst I have a sufficient number of the 1st stage, the material 

 for stages 2—4 is still too small, but of the 5th Stage I have a very large material, 

 which is able to show that no essential reduction takes place in stages 2, 3 and 4. For 

 the 6th and end of the 5th Stages I use the measurements recently published by A. C. 

 Johansen in an interesting work' on the occurrence of the young eels in the North Sea 

 and Kattegat, based on the investigations he made in the "Thor" during the spring of 

 1905. This will work be cited later; I need only note here that Johansen was able to 

 confirm Grassi and Calandruccio's observations, that the unpigmented eel young which 

 occur in the sea early in the year are larger (longer) than the pigmented which are found 

 later in the year in fresh vyater. At the same time Johansen shows that the colouriess 

 (unpigmented) eel young do not feed. These interesting observations show therefore, 

 that the metamorphosis in the colourless young of the eel found in the North Sea and 

 Danish waters in March — April is not yet complete. 



I may now mention the conclusions to be drawn from my material with regard to 

 the reduction in length during metamorphosis. 



Measurements are given on the accompanying table (p. 172) of the eel larvae and 

 young from June, January, March — April and June (in millimeters). 



I represents the measurements of the specimens of the 1st Stage {L.brevirostris), 

 which we took to the west and south of the British Isles in June 1905. 



I A. C. Johansen, Remarks on the Life History of the Young Post-Larval Eel (Anguilla vulgaris Turt.); 

 Meddelelser Kommis. Havundersagelser, Serie Fiskeri, Bind i, Nr. 6 (received July 1905). 



