STUDIES ON THE PROTOOHORDATA. 317 



on the left side, in B. Kupfferi, as is w^ell known, there are 

 two such pores — a right and a left. 



Returning to the question of the dorsal position of the 

 mouth in the Ascidian tadpole, T have on a previous occasion 

 (see this Journal, vol. xxxii, N. S., 1891, pp. 214 — 217) put 

 forward the suggestion that the lateral position of the mouth, 

 and consequently the unilateral position of the gill-slits, in the 

 larva of Amphioxus, was due to th"e mouth having been shifted 

 from a primitively dorsal to a lateral position by the secondary 

 forward extension of the notochord (see PI. 12). Any attempt 

 to account for this position of the mouth on principles of 

 utility to the larva would be futile, because it only occurs 

 during the period in which the larva is pelagic. On the other 

 hand, when the young Amphioxus begins to burrow in the 

 sand at the bottom or near the shore, frequently lying on its 

 side on the sand, the mouth has already become median, an- 

 teriorly directed and ventral. The observations which I have 

 been able to make as to the relations existing between the 

 mouth, hypophysis, and nervous system in the Ascidians have 

 raised the above view as to the origin of the asymmetry of the 

 larva of Amphioxus in my mind from the rank of a tentative 

 suggestion to that of a demonstrated fact. 



In the Ascidians, as we have seen, the neuropore opens, or 

 more correctly reopens, at first directly into the stomodseum. 

 Later on there is some reason for supposing that an evagina- 

 tion occurs from the stomodseum which carries the original 

 neuropore further back. 



In Amphioxus the neuropore opens for a long time directly 

 to the exterior in the dorsal middle line, and then later an in- 

 vagination of the epidermis occurs, which carries the neuro- 

 pore some distance inwards. This invagination gives rise to 

 the so-called " olfactory pit " of Kolliker, or " Flimmergrube " 

 of Hatschek, and into its base, as shown by Hatschek, the 

 nerve-tube at first opens by the neuropore. Eventually the 

 neuropore becomes closed, and the olfactory pit is then a ciliated 

 cul-de-sac abutting against the anterior end of the nerve-tube. 



Thus the so-called olfactory pit of Amphioxus bears precisely 



