THE ASYMMETRY OF THE LARVA. 87 



several epithelial layers; any cellular elements it may obtain 

 being derived by migration from these epithelial layers. 



9. The Asymmetry of the Larva. 



The asymmetry of the larva during its early stages is one of 

 the most striking features in the development of Amphioxus. 

 The fact that, but for the alternation of the myotomes on the 

 two sides of the body, the embryonic stages are symmetrical ; 

 and the further fact that at the close of the larval period the 

 symmetry is regained, indicate that the asymmetry of the 

 earlier larval stages is a secondary or acquired character, and 

 that the explanation of it is probably to be found in peculiari- 

 ties of habit or environment of the larva during these stages. 



The cardinal point in the asymmetry of the larva is the 

 lateral position of the mouth, which, coupled with its huge size, 

 is probably sufficient to explain the displacement of the gills of 

 the left side. 



Willey has suggested that the lateral position of the mouth 

 is correlated with, or actually due to, the anterior extension of 

 the notochord. The mode of development of this front end of 

 the notochord, and a comparison with other Vertebrates, stronglv 

 suggest that the prolongation forwards in front of all the 

 other organs of the head is a secondary feature, associated not 

 improbably with the burrowing habits of Amphioxus ; and if 

 we assume that the ancestral mouth was, as in the Ascidian 

 tadpoles, dorsal in position, then the forward growth of the 

 notochord would of necessity cause lateral displacement of the 

 mouth. The suggestion is an ingenious one, and may be 

 accepted as at any rate a provisional explanation. 



III. THE ADOLESCENT PERIOD. 



At the close of the larval period, i.e. at the completion of 

 the critical stage, the young Amphioxus abandons its pelagic 

 habits and burrows into the sand, where it passes the rest of its 

 life ; burying itself upright, with the tail downwards and the 

 buccal hood alone projecting from the sand. 



The further development takes place gradually. There is 

 a steady increase in size, but no new myotomes are formed, the 

 full number being present at the critical stage. The gill-slits, 



