POLYZOA. 295 



and muscles. From the mesoblast there is also formed, ac- 

 cording to Hatschek, a pair of ciliated excretory canals, in the 

 space between the mouth and anus (fig. 130 B, npk.). The 

 development of the nervous system has not been observed. 



At a comparatively late stage in the development there is 

 formed round the edge of the vestibule a ring of long cilia (fig. 

 1306, m.). 



A remarkable organ makes its appearance on the dorsal side 

 of the oesophagus (the side opposite the adult ganglion) formed 

 of an oval mass of cells attached to the epiblast at the apex of a 

 small ciliated papilla (fig. 130 A and B, *.). This organ will be 



FlG. 130. TWO STAGES IN THE DEVELOPMENT OF PEDICELLINA. (After 



Hatschek.) 



v. vestibule ; m. mouth ; /. liver ; kg. hind-gut ; a. anus ; and. anal invagination ; 

 nph. duct of kidney \ fg. ciliated disc ; x. dorsal organ (probably bud). 



spoken of as the dorsal organ. According to Hatschek it 

 develops as a solid outgrowth of the hypoblastic walls of the 

 mesenteron shortly before the mesenteron joins the oesophagus 

 (fig. 129 B, jr.). The cells composing it arrange themselves as a 

 sack, which acquires an external opening on the dorsal surface 

 (fig. 130 A, jr.). In a later stage the lumen of the sack dis- 

 appears, but at the junction of the organ with the epiblast a pit 



