436 THE EVOLUTION OF THE METAZOA 



their visible function as motors of the coelom fluid, of the 

 circulation— of the so-called rosettes that can be found in the 

 Ctenophora which I (Hadzi, 1957) have tried to explain as 

 remains of the terminal organs of the protonephridia that had 

 grown together with the gastral epithelium. These could thus 

 be some kind of analogous formations. The ancestors of the 

 Sipunculida possessed many pairs of the metanephridia that 

 evolved from the protonephridia. These metanephridia have 

 generally become superfluous as a consequence of a greatly 

 changed way of life. It is the anterior pair only or a few 

 individual metanephridia (in one species of the Sipunculida 

 we can even see that all the metanephridia have been lost) 

 that have been preserved in a somewhat changed form in 

 order to serve above all as gonoducts. The sexual cells are 

 soon freed in the Sipunculida from their gonads, they circulate 

 in the coelom fluid which functions at the same time as blood, 

 and it was in this way that the metanephridia had been changed 

 into gonoducts. This change had actually taken place as early 

 as in the Annelida. The nephrostomes of the remaining me- 

 tanephridia had undergone a considerable transformation; 

 they evolved first into the "sitting" and later into the freely 

 swimming urns. At the same time the individual cells have 

 preserved a strong ciliation. This seems tome to be a very 

 probable interpretation which can also be used as a working 

 hypothesis. 



The richly developed net of canals are either of the 

 coelomatic or of a schizocoelic origin and which serve as a 

 "hydraulic machine" of these remarkable animals must be 

 considered as a special "invention" of the Sipunculida. 



Finally, we must not leave unmentioned the fact that solid 

 forms can be found in their "coelom blood." Baltzer 

 (1931:269) wrote about these forms as follows, "Die Mannig- 

 faltigkeit aller dieser Zolomgebilde gibt dem Leibeshohlen- 

 inhalt der Sipunculiden eine erstaunlich hohe Komplikation, 

 cine Kompliziertheit, die von keiner anderen Tier-Gruppe 

 erreicht wird." I mention this circumstance because it again 



