No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 253 



Fig. VIII. — Diagrams of subumbrella showing the lateral extension of the borders of the somatic 

 plate on either side and their final concrescence in the mid-ventral line. The point / is the 

 posterior end ; hi., the blastopore ; the lines a, b, c, d, the ventral border of the somatic plate. 

 The material at p does not move, but that at a, b, c, d, on either side, moves round as the 

 blastopore closes, and meets that of the other side in the mid-ventral line. As a result the 

 point p is entirely surrounded by cells of the somatic plate. 



The figures on Plates III, IV, V show the various stages in 

 the lateral extension of the somatic plate. 



