- 258 - 



Leptocephali, usually obtained for investigation namely from the surface layers, is so 

 remarkably welldefined. 



Leptocephalus brevirostris is found in the Atlantic from May onwards through the 

 summer to September but as no observations earlier than May are to hand we do not 

 know in what month the Leptocephalus stage first appears. All the "Thor's" specimens 

 from June 1905, namely ca. 265 specimens, are in essentially the same stage of devel- 

 opment, that which is the highest point of the larval development and is 

 characterized by the features that the feeding stage is over and the retrogressive meta- 

 morphosis not yet begun. The animals no longer take food at this stage, which also 

 applies to other Leptocephali examined by me in this regard (cf. above). With perhaps 

 a single exception all our specimens of Leptocephahis brevirostris are thus fuU-grow^n, 

 and their average length is a little over 75 mm., the variation in length being 

 from 60 to 88 mm. In addition to the length the specimens vary somewhat in breadth 

 (height), as can be seen from the photographs of Plate VII. A row of characteristic larval 

 teeth is present both in the upper and under jaws, the anterior of which have the character 

 of grasping teeth (Fig. 6 A, p. 170). Pigment is quite lacking except in the eyes, the iris 

 of which is silvery. The anus lies far back, so that the distance from the anus to the 

 end of the tail is about 73rd of the total length. This stage, the highest point of the 

 larval development, is called by me Stage l in contrast to the later metamorphosis stages. 



In addition to the Stage l mentioned Leptocephalus brevirostris, the 1905 invest- 

 igations of the "Thor" have also given us all the transition stages between this and the 

 colourless elvers, which have been known for many years from the shallow waters on 

 the West and North European coasts and which in this work have been denoted as the 

 developmental Stage 5. The intermediate (2nd— 4th) stages were found, like the first over 

 deep water in the Atlantic but later in the year at the close of summer and in autumn. 

 It appears from the "Thor's" investigations that the metamorphosis of the eel 

 larvae takes place in the autumn out in the Atlantic, where the larvae 

 during metamorphosis live pelagically. Plate VIII gives photographs of all the 

 various stages of metamorphosis, from which it will be seen that there is good agreement 

 in the main between my specimens from the Atlantic and Grassi and Calandruccio's 

 from the Mediterranean ^ 



The accompanying Table (p. 259) summarises the characteristics of the various stages 

 in the metamorphosis and their biological conditions. 



By metamorphosis of the eel larvae I mean the whole process of change, during 



I There is this difference however, that 1 have always found that the anus normally moves forward almost 

 to the middle of the body before the height is essentially reduced, whereas the opposite might seem to be the 

 case to judge rom Grassi and Calandruccio's Fig. 2. (Cf. p. 168, note). 



An observation made during the cruise of the "Thor" in 1906, however, obviously gives the explanation 

 of this apparent disagreement. It appeared namely that when I kept even very broad specimens of the ist Stage 

 (May and June) in an aquarium these became in a few weeks much narrower without otherwise showing the 

 least sign of beginning metamorphosis. I ascribe this to the abnormal circumstances under which they lived 

 in the aquarium and consider it therefore as abnormal. (Cf. GuASSi and Calandruccio's figures 2 and 3, I. c. 

 p. II — 12, of which the first just comes from a specimen which has been living for a long time in captivity.) 

 (Note added during press.) 



