retaining its old position, has grown in upon the 

 ventral 3isk toward the mouth. Fig22.cb 3. On the 



ventral side of the larva tho fourth band has shift- 

 ed from its 'old positio; behind the third radial 

 canal to one on the interradii between arms II and 

 III, and III and IV. It has also become interrupted 

 on the oral dish in a manner similar to the third 

 ciliated band. Fig. 22. cb 4. 



The depressions, before refered to, are caused 

 by the invagination of the nervous system which 

 structure has been forming since "D"in the thicken- 

 ed oral dish of ectoderm. Immediately below' the water 

 ring and radial water canals, the thickening 'has in- 

 creased more rapidly than at ether points *,thus pro- 

 ducing a ring shaped ridge fro™ which extend five 

 re.di^l thickened ridges. These rudiments of the 

 nerve ring anc radial nerves bulge inwardly, no ev- 

 idence of their presence being apparent en the out- 



. When the thickening process has been complet- 

 ed, tl ■ system gradually sinks in , 



circular groove from which 4rfes five radial 

 grooves pass out. This is the stage in the formation 

 of the nervous system which has been reached in "F". 



