576 



row soon remove Icinporarily to tlie right-side of the larva, Tliej 

 open freely outward and not in a gill-cavity or atrium, wliicli is only 

 formed during the metamorphosis by the fusion of a longitudinal 

 fold, which lias formed itself, during the larval growth period, on the 

 left-side of the body ovei- the gill-slits with a similar fold, whicii has 

 developed itself on the right-side of the body. 



During the metamorphosis appears likewise the right row of the 

 gill-slits — 8, rarely 7 or 9, in number— which do not open directly 

 outward but in the gill-cavity. The slits of the left side, whicli had 

 temporarily removed to the right side, return now to the side to 

 which they belong. 



I can confirm Willey's observation, that the first left gill-slit 

 aborts, and that also the 10^^' to the 16^'' disappear during the 

 metamorphosis. At the end of the metamorphosis the young animal 

 is then symmetrical with regard to the gill-slits, and there are 8 of 

 them on the left-side corresponding with the eight on the right-side. 

 It is of secondary importance that the symmetry is somewhat oblique; 

 every left gill-slit does not lie exactly directly opposite the right 

 one, but half the width of the *lit more rostral. A similar oblique 

 symmetry is likewise shown by the nerves and muscles of the body 

 of the left-side comi)ared with those of the right-side of the animal. 



With the exception of the foremost slit, whicli remains undivided, 

 as long as the animal lives, each slit is divided into two parts, in 

 a longitudinal direction, by a clas}) or "tongue" growing from the 

 dorsal rim, till it reaches the ventral I'im with which it fuses. 



Directly after the metamorphosis the animal possesses thus, both 

 on the right and on the left side, a row of 8 (rarely 7 or 9) gill-slits. 

 During tlie rapid growth that follows now. this number regularly 

 increases during the whole life-time of the animal, because constantly 

 a new pair of slits develop themselves at the hindmost part of the 

 gill-basket. 



But after the metamorphosis the mouth ') seems to be a symmetrical 

 organ ; it is no longer situated distinctly on the left-side of the body 

 as in the larval growth-period, but more ventrally and almost hahed 

 by the median plane as with all vertebrates. 



The symmetrical placing of the glll-sUts is real ; since 1893 1 have 

 demonstrated however that the symmetrical placing of the mouth of 

 Amphioxus is only so in appearance. In reality the mouth, also of 

 the full-grown Amphioxus, is an organ of the left-side; for its inner 

 parietes are exclusiNely i)rovided for by nerves of the left-side, and 



^) Not to be mistaken for the mouth of the larva, sec the conclusiou of this article. 



