214 ARTHUR WILLEY. 



along the whole length of this ridge, as they do also along the 

 upper fold. 



It has been stated above more than once that the lower fold 

 of the oral hood in the larva becomes the right half of the 

 oral hood in the adult ; and this half is, as Professor Lankester 

 has pointed out (5), continuous in front round the extreme 

 point of the body with the dorsal fin. 



A little reflection will show that this is the condition that 

 might be expected from the development of the lower fold, as 

 shown in the drawings accompanying this paper. 



It is important to note that the buccal skeleton grows at 

 each end only, and that fresh elements are not formed inter- 

 stitially. In the adult the median cirri are smaller than the 

 others ; and one would at first naturally suppose that these 

 were the youngest, and that this was the point at which fresh 

 cirri would be formed; as a matter of fact, however, the small 

 size of the median ventral cirri of the adult is deceptive, for 

 they are the oldest cirri, and new ones are only added at the 

 free extremities, right and left, of the buccal skeleton. 



General Considerations. 



It is my intention to confine myself mainly to the attempt 

 to give some explanation (or at least suggest one) of the three 

 most prominent features in the larva of Amphioxus, namely : 



1. The asymmetry. 



2. The endostyle. 



3. The club-shaped gland. 



The Asymmetry of the Larva. 



With regard, then, first to the asymmetry of the larva, we 

 must begin by assuming — and the assumption will be sup- 

 ported later on — that in the primitive type from which the 

 Cephalochorda diverged the notochord did not extend to the 

 anterior end of the body, but, on the contrary, that the forward 

 extension of the notochord in Amjjhioxus is secondary and 

 adaptive ; and further, that tiie primitive position for the 



