POLYCHAETA 



251 



third quartette). During this pelagic existence the rudiments of the 

 aduh worm continue their development — which is best traced in 

 Polygordius — the apical organ develops into the prostomium of the 

 adult with brain, tentacles and eyes, while the trunk rudiment formed 

 by the proliferation of the ventral plate and the mesoblast cell grows 

 backwards as an ever-lengthening cylindrical process containing the 

 end gut. In the ectoderm of this is developed ventrally the rudiment 

 of the ventral nervous system, while to the sides of this and internally 

 are the mesodermal strips (derived from the single cell d^), which 

 show at once metameric segmentation (Fig. 187), first as pairs of solid 

 blocks, then with cavities, to form the somites. Each of these box- 

 like mesodermal segments has then an inner wall which is applied 



inesc. 



oes. 



mes. 

 an.v. 



Fig. 186. Trochosphere larva of £'z/^owafz<5. Side view. After Shearer, a/). o. 

 apical organ; e. eye; prt. preoral ciliated ring; hk. "head kidney", larval 

 nephridium ; otc. otocyst; mes. mesodermal band ; an. anus ; an.v. anal vesicle ; 

 hl.c. blastocoele ; mesc. mesenchyme ; oes. oesophagus ; st. stomach. 



to the gut (splanchnic mesoderm) and an outer (somatic mesoderm) 

 lying under the ectoderm. The right and left rudiments meet in the 

 middle lane and are only separated by the dorsal and ventral mesen- 

 teries which are formed by their apposed walls, while the anterior 

 and posterior borders of each segment are septa. At the same time 

 the adult nephridia develop from ectoderm rudiments and the blood 

 vessels differentiate in the septa and mesenteries. 



The advanced larva (Fig. 187 A) thus consists of two rudiments of 

 the adult body, separated by the body of the larval trochosphere. 

 They are joined by a pair of longitudinal muscles and of nerves, and 

 in one species of Polygordius metamorphosis of the larva into the 

 adult is brought about by the shrivelling up of the larval tissues and 

 the drawing together and the union of the head and trunk assisted by 



