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 



vr- 



vtv- 



FIG. 111. Longitudinal sections of anterior portion of Polygordius nenpolitfiniis immediately 

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



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

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

 cells surrounding oesophagus; schiz, schizocoelic space in front of brain; 6.r, blood ring; r.7, ventral 

 blood-vessel ; we.*, 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 



