THE NEW GENEALOGICAL TREE 433 



above all in the Sipunculida. As an introvert it can even be 

 longer, at least in some cases (Phascolion, Aspidosiphon, Oncho- 

 soma), than the length of the actual body, longer than the 

 postsoma. This proboscoid introvert had developed in the 

 fully sessile Ectoprocta into the so-called polypid; the postsoma 

 on the other hand is called here a cystid. 



A polypoid develops from a "worm;" one of the most re- 

 markable features of this development is a radially-symmetric 

 circle of tentacle -like bodywall extensions which had evolved 

 around the oral opening and which show in some groups of 

 the Oligomeria a more bilateral arrangement (in the shape of a 

 horseshoe, Phoronida, Brachiopoda, a considerable part of the 

 Ectoprocta). A group of tentacles arranged approximately in 

 the shape of a horseshoe can be found already in some species 

 belonging to the Sipunculida. 



The fact seems to me to be very important that as early as in 

 the Sipunculida we can find a kind of an initial formation of a 

 calcified exoskeleton as this is best shown by the case of 

 the Aspidosiphon speciosus (according to Hyman). On the one 

 hand we can find here a "posterior shield" secreted at the 

 posterior end of the postsoma which can be compared with an 

 ectocyst, and on the other hand there appears another pro- 

 tective layer at the basis of the introvert, at the place where 

 a protective plate (operculum) can be found in the Ectoprocta, 

 and which corresponds to the collar region, or rather sub- 

 region, of many Oligomeria. The introvert of the Sipunculida 

 evolved into a very important organ which is no homologue to 

 the proboscis of the Echiuroidea. To this we must also add the 

 strongly developed retractors that can be frequently found 

 in the Oligomeria. The introvert is frequently covered with 

 various protuberances, even spines, which can be considered 

 as a specialization of the Sipunculida. 



A characteristic pecuUarity of the Sipunculida is the fact 

 that the place of their anal opening, which had been transferred 

 frontwards, has not been completely stabilized. It usually occurs 

 dorsally at the upper end of the postsoma; in the Onchnesoma 



