579 



of llie so-called praeoral pit, whicli is foriiied as a sejoiiied pai't of 

 (he pharynx and soon opens to the ontside. 



In accordance with this view is the place where the thyroid 

 gland of Amphioxus originates. In all vertebrates this gland is formed 

 as a median outgrowth of the epithelium of the pharyna immediately 

 behind the mouth, between the tirst pair of gill-slits, if they are 

 taken as fused with their ventral extremities. If now we see in the 

 mouth of Amphioxus the homologue of that of vertebrates, then, in 

 an incomprehensible way, the thyroid gland of Amphioxus witnld 

 be formed in front of the mouth instead of behind it. 



What explanation can now be given of the fact, that Amphioxus 

 has lost its primitive mouth and has obtained secondarily the first 

 gill-slit as mouth, whilst in the stage of the larval growth — now 

 letting alone the club-shaped gland — not the gill-slits of the right- 

 side, but only those of the left-side open to the exterior and moreover 

 in the median plane, whilst they even partly i-emove temporarily 

 to the right-side? 



The key to this explanation is, in my opinion, to be found in 

 the movement of the young embryo which has been observed by 

 Hatschek. This embryo moves, turning on its longitudinal axis, 

 helicoidally forward ; the rotation takes place from right to left. 



If now one admits that ancestors of Amphioxus have moved 

 forward in this way, to which they may have been induced, because 

 they missed an auditi\e or equilibrium-organ, the trace of which 

 does not even appear in Amphioxus, then it is to be understood 

 that the left first gill-slit must have had the predominance o\er the 

 niedianly placed |n'imitive mouth as opening for the admittance of 

 water, which must sei-ve at the same time both for respiration and 

 for nutrition. The following gill-slits had to evacuate the res|)iiali()u- 

 watei-, but this evacuation was for the slits on the left-side ijnpedcd 

 by the way of mo\ing of the animal. On account of the rotation 

 from right to left, the following gill-slits on the left-side would be 

 inclined to ingest water instead of evacuating it, and therefore 

 they were obliged to remove from this side to the median plane, 

 or still better to the i-ight-side, where the evacuation of the resj)i- 

 ration-water was exactly facilitated in consequence of the movement. 



By this removal, however, came the original gill-slits of the i-iglit- 

 side in a tight j)lace ; they remained little, and this is the i-eason 

 why in the Anq)hi()xus-larva, the} api)ear only in the j)eriod of the. 

 metamorphosis. 



When later ancestors of Amphioxus gave up their swimming way 

 of li\ing and buried themselves into the sand, as he does still now, 



