The Development of Phascolosoma. J47 



E. Meyer's view that the blood vascular system of the Sipunculids 

 corresponds to the anterior body-cavity of the Chaetopods, or to the 

 cavity of the lophophore of Phoronis and the Bryozoa, or to the 

 hydrocoel of the Echinoderms. 



Conclusions. That the Sipunculids are probably primitive 

 forms is shown by their unpaired, unsegmented, ventral nerve cord, 

 by the retention in the adult of the principal retractor muscles of 

 the^ trochophore, and of the single pair of thoracic nephridia. The 

 entire organization of the adult is exceedingly simple and trocho- 

 phore-like. 



The transitory metamerism of the trochophore of Ph. goiddii is 

 not necessarily to be interpreted as a sign of degeneration from a 

 fully segmented type, but it may, and probably does, indicate merely 

 a near relationship to that type, an incipient tendency toward meta- 

 merism. 



Sipunculids are Annelids that are closely allied to the Chaeto- 

 pods and to primitive molluscs, but are even simpler in structure 

 than the Archiannelida. 



13. Appendix. 

 A. Generic characters of Sipunculus and Pliascolosoma. 



The name Phascolosoma goiddii Kefeestein has been employed 

 in this paper instead of Sipunculus gouldii Pourtalès, because the 

 basis of distinction between these two genera, which was made by 

 Selenka (1883—1884) and followed by Andrews (1890), breaks down 

 when applied to this species. That is to say, the mere division of 

 the longitudinal musculature of the body wall into distinct bands 

 is a wholly insufficient reason for separating Ph. goiddii from its 

 close ally Ph. vulgare, which it strikingly resembles in external 

 features and internal structure, not only in the adult condition, but 

 also throughout the course of its embryonic and larval development. 

 On the other hand Ph. gotddii and Ph. vulgare dilfer widely from 

 S. nudus in external features, in several points in internal structure 

 (vide Ward, 1897, p. 177), and especially in their development. 

 A thorough revision of the Sipunculidae is needed, which shall take 

 into consideration embryological facts, such as the difference in 

 structure and fate of the prototroch, as well as fundamental differences 

 in adult structure. 



10* 



