LATER LARVAL DEVELOPMENT OF AMPHIOXUS. 183 



The Later Larval Development of Amphioxus. 



By 



Artbur TVilley, B.Sc. 



(Of University College, London). 



With Plates XIII— XV. 



In the account of the development of the atrial chamber of 

 Amphioxus which appeared in this journal last August, under 

 the joint names of Professor Lankester and myself, it was 

 stated that we had not been able to trace the progressive steps 

 in the history of the gill-slits, during their passage from the 

 unilateral position which they hold in the larva, to the sym- 

 metrical condition which we find in the adult ; nor the trans- 

 lation of the lateral mouth of the larva to the anterior median 

 position of the mouth in the adult. 



We added that we had taken steps to obtain the critical 

 stages in the living condition during the summer of last year 

 (1890). I accordingly went down to Messina, for the second 

 time, in July, to undertake the study and to make drawings of 

 the living larvae — being enabled to do so by the continued kind- 

 ness of Professor Lankester, to whom for this, as also for invalu- 

 able assistance and advice since received, I am deeply indebted. 



According to Professor Lankester's instructions I spent the 

 months of July and August at Faro, daily fishing in the 

 Pantano, and concentrating my attention on larvae of the 

 particular stages which were required. To obtain a large 

 number of individuals in the desired condition of transition, 

 from the extreme asymmetry of larval life to the nearly sym- 

 metrical condition which is reached with scarcely appreciable in- 

 crease of size, was no easy matter. By examining daily a great 

 number of larvae I succeeded in obtaining an ample series of the 



