VII 



ANNELIDA 



151 



layer become nerve cells, whence nerve fibres run along the pharynx 

 in a longitudinal direction (oes.m, Fig. Ill, A). 



When the animal has stripped off its prototroch it sinks to the 

 bottom : the eye-pits become functional, but their principal use is to 

 avoid the light. When eventually the adult adopts a burrowing life 

 the eyes generally degenerate. 



In the North Sea species, P. lacteus, the later development is 

 somewhat different from that which we have just described. Thus, a 



Fig. 111. — Longitudinal sections of anterior portion of Polygordiws neapolitanus immediately 

 after metamorphosis, to show details. (After Woltereck. ) 



A, frontal section at level of oesophagus and intestine. B, frontal section dorsal to intestine. 0, 

 sagittal section. Letters as before. In addition, VI, dorsal blood-vessel ; oes.m, mantle of ectodeim 

 cells surrounding oesophagus ; schiz, schizocoelic space in front of brain ; 6.r, blood ring ; v.l, ventral 

 blood-vessel ; mes, wandering cells in the blastocoele. 



ring-shaped invagination of the thin ventral surface of the larva is 

 formed, giving rise to a circular fold, which we shall term the 

 amniotic fold, surrounding the trunk blastema; and the space between 

 this fold and the ectoderm covering the trunk blastema — in a word, 

 the cavity of the invagination — grows forwards and upwards in the 

 form of four pockets, a dorsal, a ventral, and two lateral. 



When further growth and cell division begin in the trunk 

 blastema the new post-trochal body thus formed is folded up within 

 the cavity covered by the circular fold just mentioned, and only at 

 the moment of metamorphosis is the body straightened and thrust 



