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1014 



and swims like all other fishes with its back up, the side-planes 

 turned to riglit and left. 



Later on however the animal swims with one side-plane turned 

 lip and the other down. The eye of the wiiite imder-surfaee then 

 removes to the pigmented upper-surface, where the other eye is 

 already situated. 



An analogous explanation of the situation of the mouth in Am- 

 phioxus is impossible, because it becomes clear through the nerves 

 and muscles of the displaced eye in the Pleuronectidae that this 

 belongs to the under- and not to the upper-surface, on which it has 

 come to lie. 



The nerves and muscles on the inside oftlie mouth of Amphioxus, 

 which all come from the left side, prove on the contrary, that it 

 cannot be considered as a mesial organ, but in fact belongs to the 

 left side. 



While the mouth is formed on the left side in the young larva, 

 a glandular metamorphosed gill-pouch, known as the club-shaped 

 gland, arises opposite to it on the right side. 



Its discoverer, Hatschek (1881), knew the external opening of 

 this gland, but not yet the intestinal opening. Its presence was 

 established by later investigators. 



It was now clear that the mouth of the larva of Amphioxus could 

 be considered as a metamorphosed gill-pouch, and as an antimere of 

 the club-shaped gland. 



The question however arises with which organ of the higher 

 animals the left-sided mouth of the Amjihioxus larva is homologous. 

 Is it the homologue of the left half of the mouth of the Craniota, 

 that would then have to be considered as a product of fusion of 

 the mouth of Amphioxus with the club-shaped gland ; or is the 

 mouth of the Amphioxus larva the homologue of the first left gill 

 slit of the Craniota, which is known in the Selachii as the left 

 spiracle? 



Various grounds led to the conclusion that the hitter must be the 

 case. To my mind this was proved when it was found that the 

 body-cavity of the lower jaw, the mandibular cavity, does not in 

 Amphioxus lie behind the larval mouth, as is the case in higher 

 animals, but in front of it. 



Amphioxus, indeed has no jaws, upper- as little as lower jaw, 

 but still it has a mandibular cavity that indicates the position of 

 the lower jaw. It is self-evident that an opening, arising behind the 

 place of the lower jaw, cannot be considered a homologue of a 

 part of the mouth of the Craniota. 



