302 



THE INVERTEBRATA 



which there appear to be as many as fifteen pairs of ganglionic 

 swelHngs (Fig. 208). 



Echiurus, with a spoon-shaped prostomium, two pairs of segmental 

 organs and a trochosphere larva. 



Bonellia (Fig. 209 A, B) with a prostomial proboscis bifurcated at 



m. 



mes. 



Fig. 208. Echiuriis. Ventral view of larva to show segmentation of posterior 

 end. After Baltzer. a. anus ; ch. chaeta-forming cell ; com. neural commissure ; 

 l.m. longitudinal muscle; m. mouth; rnes. mesoderm ; /)w. larval nephridium 

 with solenocytes ; vn. ventral nerve cord composed of many neuromeres. 



the end, capable of enormous elongation and extremely mobile; a 

 single segmental organ (brown tube) ; the female is the typical in- 

 dividual and the males are reduced to small ciliated organisms, like 

 a turbellarian, which live in the segmental organ of the female. 



It is now known that larvae of Bonellia carry the potentialities of 

 both sexes. If they develop independently they become females. If 



