1 62 DEVELOPMENT OF AMPHIOXUS. 



symmetrical to a secondary asymmetrical position, as is the 

 case, for instance, with the neuropore. 



It may appear paradoxical, but is nevertheless correct, to 

 say that in the ontogeny the mouth and gill-slits appear 

 Primarily in a secondary position. 



It is quite evident that the asymmetry of the larva of 

 Amphioxus is of a totally different character to the well- 

 known asymmetry of the flat-fishes or Pleuronectida 

 (turbot, sole, plaice, halibut, flounder, etc.). The latter 

 are hatched as perfectly symmetrical larvae with eyes quite 

 opposite to one another. Then, in adaptation to a life at 

 the bottom of the sea, after a short pelagic existence they 

 turn over on one side, in some species the right side, and 

 in others the left, and the eye of that side moves over the 

 snout, sometimes even through the snout, to the other 

 side, and so the eyes come to lie on the same side. In this 

 case, therefore, the asymmetry, which is secondary in every 

 sense of the word, is the result of a special adaptation to a 

 particular habit of life, and is accordingly of the greatest 

 advantage to the fishes which possess it. 



On the other hand, its extraordinary asymmetry is of 

 no conceivable advantage to the larva of Amphioxus, and 

 does not represent an adaptation to any peculiar mode of 

 existence whatever. 



It is rather the mechanical, incidental, accessory, and 

 subsidiary accompaniment of another organic change which 

 is both advantageous and adaptive, namely, the forward 

 extension of the notochord ; and while the excessive asym- 

 metry is indifferent to the pelagic larva, it would be posi- 

 tively detrimental to the adult. 



Thus in all respects the larval asymmetry of Amphioxus 

 is the precise converse of the adult asymmetry of the 

 Pleuronectidae. 13 



