— i67 — 



rather ot the transition stage between this and the cylindrical stage in the Atlantic west 

 of St. K'ilda on Aug. 31st — Sept. ist. Though the September material is thus much smaller 

 in numbers than the June, it is nevertheless of considerable interest in that it represents 

 all the principal stages in the metamorphosis of the eel, just those stages which had 

 previously been known only from one single place, the Straits of Messina. The invest- 

 igations of the "Thor" in September have thus proved that the metamorphosis of 

 the eel larvae takes place in the Atlantic to the west of the British Isles. 



It should first of all be mentioned, that all the 8 specimens are considerably 

 further forward in development than the June material; none of them are at 

 the same stage of development as that represented by the ca. 265 June specimens. On 

 the other hand the 8 September specimens show no small differences amongst themselves. 

 They fall into 3 groups according to their form and general appearance (see respectively 

 Figs. 3—6, Figs. 7 — 8 and Figs. 9— 10, Plate VIII). To describe the characteristics of these 

 three groups and to compare them both with the earlier stages of development (the true 

 Leptocephalus stage, Plate VII, Fig. 1 — 2, Plate VIII) and the older (Fig. 11—13, Plate VIII) 

 really means to describe the transformation of the eel larvae. I may therefore use the 

 opportunity to give a description of the principal stages during the metamorphosis (see 

 figures on Plate VIII), and should mention specially that the figures are reproductions of 

 direct photographs of the specimens. 



In addition to a number of changes in the inner structure of the Leptocephali, a 

 more detailed investigation of which would undoubtedly repay anatomists and histologists, 

 the following main changes occur during metamorphosis': 



(1) The height of the body is reduced, first in the anterior and posterior parts 

 of the body, later in the central portion ; last of all the length and thickness of the trunk 

 are also reduced. 



(2) The size of the eyes is reduced (cf. the changed mode of life from deep to 

 shallow water). 



(3) The larval teeth disappear, and the pointed snout becomes rounded. 



(4) The hindmost portion of the gut disappears. 



(5) The front parts of the dorsal and anal fins grow forward. At the 

 same time, the angle formed by a line through the anterior points of the dorsal and anal 

 fins with the longitudinal axis of the body becomes more acute. 



(6) Pigment is developed first on the end of t he tail later on the neck and 

 lastly over the greater part of the dorsal and lateral aspects. 



Further, the various stages in development can be grouped naturally into the fol- 

 lowing 6 groups: 



1 It is very interesting to note the gradual change in the nerves which pass to the rays and interspinous 

 rays in the dorsal and anal fins. Though all or at any rate the majority of the rays are present even in the ist 

 stage, yet both the dorsal and the anal fin are much shorter than in the following stages. In the ist stage we 

 see how the nerves emerge from the column far in front of even the beginning of the fins and are 

 therefore closely packed together opposite the fins themselves, In the following stages, when the front 

 part of the fins grows much forwards the nerves become shortened and more separated, and at the same 

 time their direction relative to the longitudinal axis is quite ch.inged (from being almost p.irallel or forming a 

 very acute angle at the most with this they become almost perpendicular to it). 



