200 ARTHUR WILLEY. 



opposite to the boinidary between the first and second slits of 

 the left side — and this is its final position. 



Except when the above-mentioned ninth slit is found in an 

 undoubtedly rudimentary condition, it is impossible to predict 

 whether it would eventually close or not. My observations 

 show that in the majority of cases it does close, but that, on 

 the other hand, it frequently does not. I have never found a 

 less number of primary than of secondary slits, so that when 

 nine of the latter are formed (of which instances have been 

 given above) it is certain that nine of the primary slits will 

 persist. 



On comparing fig. 12 with fig. 11 it will be found that the 

 gill-slits of the former are not quite so far advanced as those of 

 the latter, while the buccal cirri in the former have reached a 

 higher stage of development than those of the latter. This is 

 a slight variation. 



The junction of the originally independent upper and lower 

 portions of the oral hood is very distinct. 



The anterior or right wall of the oral sphincter or velum has 

 now passed so thoroughly round to the right side that it 

 cannot be inserted in a drawing of the left side. Four velar 

 tentacles make their appearance during this stage; they are 

 much clearer, however, in the next stage. 



The left ciliated (hyperpharyngeal) band is seen to join the 

 left or lower half of the endostyle ; the right or upper half of 

 the endostyle is not seen from this side naturally. 



In all the drawings of larvae belonging to previous stages, 

 from the left side, there is shown a peculiar structure, de- 

 scribed as a nephridium by Hatschek (4), and placed just 

 below and to the left side of the notochord in the region 

 between the praeoral pit and the mouth. 



It presented the appearance represented in the figures, but 

 I could not certainly detect cilia in it, and in fact was unable 

 to understand its import. It seems to possess a superficial 

 resemblance to the head-kidney of Annelid larvae (trocho- 

 spheres), but I can form no opinion as to the reality of any 

 such resemblance. Hatschek discovered that it opened at one 



