ON THE ANCESTRAL FORM OF THE CHOEDATA. 367 



the oesophagus. These two diverticula, originally symmetrical, 

 become constricted off from the hypoblast, and in their further 

 development they have a different fate, the left one com- 

 municating with the exterior by a ciliated opening, 

 which appears in the epiblast, the right one forrriing an epi- 

 thelial lining in the praeoral body-region. The left one was 

 looked upon by Kowalevsky as a special sense-organ 

 of the larva. 



Although I am not prepared for the present to furnish any 

 evidence in this direction, I would call attention to the simi- 

 larity in development between these structures and the cephalic 

 diverticulum of Nemertines. Considering the amount of dege- 

 neration which in several respects Amphioxus appears to have 

 undergone, it does not appear impossible that the left lobe is 

 really a temporary olfactory organ, the right one having entered 

 upon other functions, and having lost its original significance. 



These oesophageal diverticula of Amphioxus would stand 

 about in the same relation to the posterior paired outgrowths 

 of the oesophagus which ulteriorly give rise to the branchial 

 slits of this animal, as would the two primary larval diver- 

 ticula of Balanoglossus, giving rise to the first pair of gill-slits, 

 to the following ones successively appearing behind them. lu 

 Nemertines only one corresponding pair of respiratory diverti- 

 cula is encountered, and they may remain in connection with 

 those portions of epiblastic ingrowths which form the primary 

 constituents of a sensorial (olfactory?) apparatus in certain of 

 the higher differentiated genera in the way we have above 

 traced. 



The far reaching significance of our starting point has 

 obliged us to throw a rapid glance at the principal points in 

 which Nemertines allow of a certain degree of comparison with 

 Vertebrates, and it would lead us too far off if we were to follow 

 this up for the secondary, less important points, or for those 

 which are at present not fully enough known to allow of any 

 fruitful comparison. Among the latter I count the excretory 

 and the generative apparatus. Do the closed generative sacs 



