575 



After the expiration of tlie einhiyona! period one distingnislies 

 tliree stages in the development of Anipliioxns: l""^ the stage of the 

 larval growth; 2"'^ the stage of tiie metamorphosis; 3'^^ the stage 

 of the postlarval growth. 



At the beginning of the first stage the larva is 1 ra.m. long; at 

 the end of it it hcis reached a length of between 4 and 5 m.m. At 

 the beginning of this period only the first gill-slit is formed, behind 

 it are gradually developed a second, a third etc. till a number of 

 14 to 16 is reached. All these gill-slits belong morphologically to 

 the left side of the body; those of the right side appear only in 

 the period of the metamorphosis. During the stage of the larval 

 growth constantly new muscle-segments (myotomes) are added to 

 the posterior part of the body, but at the beginning of the meta- 

 morphosis this number is already complete. The animal is then 

 only 4 to 5 m.m. long, but it posesses already the complete number 

 of 60 muscle-segments with the nerves appertaining to them, which 

 are also found in the full-grown animal which is almost a finger long. 



During the comparatively long time of the metamorphosis, which 

 is divided by Willey into 8 subdivisions, astonishing changes take 

 place, not so much in the nervous or muscular system (with the 

 exception of the gill-muscles), but especially in the shape and the 

 placing of the mouth and of the gill-slits. 



The animal does not grow during the metamorphosis, for its 

 length amounts, both at the beginning and at the end, to between 

 4 and 5 m.m. ^). Differences in length do not indicate here a further 

 development. A larva that is half a millimeter longer than another 

 needs not be older than the latter, but is often \ounger. It has even 

 appeared to me that, during the first half of the metamorphosis, the 

 length rather decreases somewhat than increases, but the individual 

 differences are too numerous to state this phenomenon as certain. I 

 am of opinion, that the fact that the larva does not grow during 

 the metamorphosis, must be attributed to its not taking food during 

 this period. Presentlj^ I shall revert to the grounds of this. 



Before the metamorphosis both the mouth and the gill-slits lie 

 perfectly asymmetrically ; the mouth does not lie medianly and 

 ventrally as with all vertebrates, but on the left-side of the body, 

 and of the gill-slits is only the row of the later left-side extant. 

 They behave very curiously, for they do not originate on the left-side, 

 but apparently in the median plane, whilst the foremost of the 



1) Larvas from the neighbourhood of Messina are during the metamorphosis still 

 smaller. According to the statements of the authors their length amounts to an 

 average only to B'/j mm. 



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