LATER LARVAL DEVELOPMENT OF AMPHIOXUS. 215 



mouth was dorsal, as it is in the Ascidian tadpole.^ The last as- 

 sumption may appear to be the most arbitrary, but it is justified 

 by the close affinities which exist between Amphioxus and the 

 Ascidians, and which will be referred to again. It may as well 

 be stated at once that the view I am about to put forward 

 depends essentially on this way of regarding the forward exten- 

 sion of the notochord ; and the immediate reasons for supposing 

 it to be secondary are — 



i. That it does not extend to the front end of the body at its 

 first origin (see Woodcut, fig. 2). 



ii. That its forward extension is an obvious advantage to 

 Amphioxus when burrowing in the sand, which is its constant 

 occupation after a certain period of development. 



iii. The analogy of the Ascidian tadpole. 



It is clear, then, that as the notochord pushed itself forwards 

 beyond the anterior limit of the nerve-tube, the mouth would 

 be compelled to forsake its primitive dorsal position — indeed, 

 it would be forced, as it were, to one side, just as the anterior 

 opening of the nerve-tube, which is at first actually dorsal, is 

 made to assume a lateral position through the development of the 

 dorsal fin ; or, again, as the anus is similarly displaced by the 

 caudal fin. It is a singular coincidence that the anterior neural 

 pore, the mouth, and the anus are all deflected to the left side. 



The lateral position of the mouth is thus supposed to be due 

 to — or to occur in correlation with — the forward extension of 

 the notochord ; and it is evident that the same process which 

 caused the mouth to rotate from a dorsal position to its pre- 

 sent position on the left side in the larva would also by 

 implication cause structures which were originally on the left 

 side, or ventral, to pass across to the right side. There can 

 hardly be any doubt that a semi-rotation of the ancestral 

 pharynx has virtually taken place ; the only question is, 

 whether we are right in correlating it with the forward exten- 

 sion of the notochord. Supposing the ancestral mouth to have 

 been small and circular, as it is in the Ascidian tadpole and in 

 the very young larva of Amphioxus, it is possible that this 

 ' See Balfour, ' Comparative Embryology,' vol. ii. 



