LARVAL DEVELOPMENT. 147 



half of the velum is nearly but not even yet quite opposite 

 to the left half (Fig. 93). 



In the preceding stage (Fig. 82) there were several 

 additional buccal cartilages added to the first one which 

 we described. In the present stage these have begun to 

 grow outwards so as to produce small notches in the 

 integument, which is now commencing at this point to 

 form the right half of the oral hood. The left half of the 

 latter arises as a downgrowth of the integument from the 

 upper margin of the prseoral pit and its posterior continua- 

 tion, the above-mentioned ciliated groove. (Cf. Figs. 81, 

 82, and 83.) The hinder portion of this fold is at first on 



J71 



Fig. 83. Anterior portion of still older larva, from the left side, to show 

 change in shape and position of the mouth. (After WlLLEY.) 



Letters as above. The left half of the oral hood is now growing down over the 

 mouth and prasoral pit. 



a level with the dorsal margin of the mouth, and in fact 

 merges into the latter, but subsequently grows over it, 

 extending to its posterior extremity, where it meets the 

 right half of the oral hood. 



It is obvious from the above description and figures that 

 a large part of the right wall of the oral hood is derived 

 from the original wall of the snout below the praeoral pit, 

 and so an explanation is afforded of the fact noted in the 

 first chapter that the right half of the oral hood is continu- 

 ous round the anterior extremity of the notochord with 

 the cephalic expansion of the dorsal fin. 9 



