64'2 GERARDA STIASNY-WIJNHOFF 



reason is quite absent, and so it is in pelagic forms. When we 

 can, moreover, follow the development, as is the case (1) in 

 the Anopla, from stages like Tubulanus pellucidus, 

 Procarinina, and other Tubulanus species through Hubrechtia 

 to the Heteronemerteans, and on the other hand in Enopla 

 from Siboganemertes (Text-fig. 12, h) to several Monostilifera 

 (Text-fig. 12, a) and to Drepanophorus (Text-fig. 13), from 

 (2) the irregular organ with partly free and simple constituents 

 through the composite and irregular organ of several Reptantia 

 as Uniporus (3, Pi. i, figs. 6 and 7) and Drepanophorus 

 cerinus, willeyanus, indicus 15, 14), to the well- 



Text-fig. 15. 



•^ -^ cer care. 



Transverse section through the cerebral organ of Emplectonema 

 gracile after Burger (6, PI. xxvi, fig. 41). 



defined organ of D. spectabilis (Text-fig. 13), or from 

 (3) Siboganemertes (Text-fig. 12, h) through stages as Em- 

 plectonema (Text-fig. 15) and Prostoma cruciatum to 

 Prostoma cephalophorum (Text-fig. 12, a), it seems to 

 be rather probable that this organ has developed in Nemerteans 

 and has not been inherited from now extinct ancestors. The 

 great development of the dorsal brain-lobe is a characteristic 

 feature of the Eeptantia, but need not have been a possession 

 of all Poly stilif era. In other forms with a well-developed 

 cerebral organ, as in Amphiporus, the difference of the propor- 

 tions of the brain-lobes is not great, and I am rather inclined 

 to think that the development of the sac caused the different 

 structure of the dorsal brain-lobe of the Reptantia. Paired 

 r h y n c h o c o e 1 o m i c diverticula are absent in all Nemer- 



