212 ARTHUR WILLEY. 



seen with quite a surface focus ; and the whole external surface 

 of the lower wall of the mouth is beset with small cilia (figs. 

 21 and 23). 



Four velar tentacles are present in Stage VII, and this 

 number continues all through the critical period. Eventually 

 fresh tentacles arise and make up the number which occurs in 

 the adult, viz. twelve. 



5. Oral Hood and Buccal Cirri. 



The right and left (lower and upper) halves of the oral hood 

 develop independently of one another, and are also entirely 

 independent of the metapleural or atrial folds, as will be readily 

 gathered from the figures. It will be seen that the oral hood 

 is formed mainly after the completion of the atrium. Professor 

 Lankester has suggested (5) that '^ the oral hood is the prseoral 

 portion of the epipleural folds. ^^ This, of course, is now seen 

 not to be the case. 



The oral hood develops concurrently with the change of 

 position of the mouth. 



The upper portion of the oral hood arises between Stage II 

 and Stage III as a fold of the integument overhanging the 

 praeoral pit and ciliated groove. Beyond the latter it merges 

 into the upper margin of the mouth. This can be seen in 

 section by referring to the paper which has been already 

 quoted several times (8, pi. xxxi, figs. 13 and 14a). As 

 the fold becomes larger it extends to the posterior extremity 

 of the mouth, where it is met by the lower fold. Anteriorly 

 the upper fold of the oral hood is continued somewhat beyond 

 the limit of the preeoral pit, and ends as a small ridge on the 

 left side of the head region, just below the level of the 

 notochord. 



The formation of the buccal cirri is coincident with the 

 development of the lower portion of the oral hood. In the 

 adult the right and left halves of the oral hood are equally pro- 

 vided with cirri. It is, therefore, a most curious fact that the 

 buccal cirri take their prime origin in the lower or right half 

 only of the oral hood, and, after reaching a considerable stage 



