— 26o — 



which they assume the appearance of the parents simultaneously with a reduction in the 

 size of the body both in height and length. Two natural periods can be distinguished 

 in the metamorphosis. During the first it is essentially the height of the body that is 

 reduced, and the result of this is the Stage 5 which is already in the main eel-shaped 

 ("hemilarvae", "glass-eel", "transparent elvers"). This first part of the metamorphosis is 

 passed through out in the Atlantic, and it is only when Stage 5 has been reached that 

 the young elvers begin to appear at the coasts of Europe. In contrast to the earlier flat 

 Leptocephalus stages these are now strong and swift as arrows in their movements. 



During the second part of the metamorphosis it is chiefly the length of the body 

 which is reduced. In biological regard this part of the metamorphosis may be' 

 characterized as a migration-period, during which the elvers seek in towards West and 

 North Europe in order to reach to the fresh water or the shallow coastal waters, where 

 they can complete their metamorphosis and begin their future life of feeding and growing. 

 The metamorphosis may be completed either in fresh or salt water. The whole of the 

 metamorphosis has thus lasted about one year', during which time the animals have 

 taken no food, but have thus, in addition to the considerable reduction in height, 

 become reduced on an average ca. l cm. in length (namely, from ca. 75 to ca. 65 mm.). 

 (Cf. p. 172.) 



The metamorphosis can be sharply defined both in morphological and phy- 

 siological regard, namely, (1) by the total reduction in length, which in any given 

 individual is determined by the difference between the greatest length (in Stage 1) and 

 the smallest of all reached in reduction (Stage 6), and (2) by their taking no 

 food during this period. 



The reason why the early stages in metamorphosis have so long remained unknown 

 off West Europe, until the scientific investigation steamers found them, is that the eel 

 larvae complete the first and most apparent part of their metamorphosis out in the Atlantic 

 and do not appear at the coasts until they have reached the eel-shaped Stage 5^ 



In the autumn, in the beginning of November, however, as appears from the collec- 

 tions of the Irish investigation steamer "Helga" in the Atlantic west of the British Isles, 

 small elvers in Stage 5 are found on their way in towards the coasts 3. And from November 

 onwards or even earlier it is no longer necessary to go out to sea to procure the 

 youngest eel fry existing at that time, as these have then begun to appear at several of 

 the West European coasts, where they run up the rivers or other outtetss. 



The distribution of Stage 5, the colourless glass-eels, in the sea during the winter 

 half-year is represented on the Chart PI. XI. The Chart shows: (i)that the eel-young at 

 this stage and this time of year are no longer restricted in their occurrence to 

 the Atlantic west of Great Britain and France, as was the case with the earlier 



1 In tlie countries (e. g. Spain and France), which lie close to the spawning places the metamorphosis may 

 perhaps be canied through more quickly, as the elvers here can enter the fresh water earlier in the year. 



2 That this is the case I have been able to determine by an investigation of 3000 tiny eel fi-y taken at 

 the period of New Year 1906 at Bayonne in the Bay of Biscay. None of these elvers were still at Stage 4; 

 they were all without doubt in Stage 5. 



3 The small elvers appear already in the north of Spain in the month of October- or September, whereas 

 they have not been observed at Ireland before November. 



