366 EMBRYOLOGY 



head portion sinks into complete insignificance (Fig. 159, 

 p. 362). As regards the formation of the embryonal 

 membrane, this might be a later acquisition, especially 

 since it is said to be wanting in Phascolosoma. Moreover, 

 Selenka argues that the pairs of so-called setae in the 

 latter form might indicate a segmentation ; but this 

 evidence cannot be considered as conclusive. 



Finally, it should be stated that the structure and develop- 

 ment of the Sipunculidce do not disprove relationships to the 

 Annelida, but that as yet no justification exists for uniting 

 them with these. We place them here next to the Annelida, 

 because definite relationships to any other branch of the 

 animal kingdom are not demonstrable and because in the 

 shape of their larva they are most nearly related to the 

 Annelida. A closer relationship to Phorcniis and the Mollus- 

 coidea appears to us not yet sufficiently established. 



Literature. 



1. Hatschek, B. Ueber Entwicklung von Sipunculus nudus. Arb. 



Zool. Inst. Wien. Bd. v. 1884. 



2. Selejjka, E. Eifurchung und Larvenbildung von Phascolosoma 



elongatum. Zeitschr. wiss. Zool. Bd. xxv. 1875. 



3. Selenka, E. Die Sipunculiden. Wiesbaden. 1883. 



4. ScHAuiNSLAND, H. Die Excretions- und Geschlechtsorgane der 



Priapuliden. Zool, Anzeiger. Jahrg. ix. 1886. 



