144 THE EVOLUTION OF THE METAZOA 



for the final elimination of the canal system which had become 

 superfluous. Thus the protonephridia were replaced by an 

 auxiliary apparatus a development which has later also taken 

 place in other groups of Eumetazoa. 



A question could perhaps appear justified: is it more proba- 

 ble that the protonephridia of Turbellaria had been developed 

 from the nephrocytes as probably the most primitive excretive 

 organ of Cnidaria? Has anything at all been known which 

 could point to such a direction? 



The Spongiae appear, as in so many other instances, in 

 connection with the nephridial system, as standing apart from 

 all other genuine Metazoa, the Eumetazoa. In the Spongiae 

 we find that not only are nephridia absent (this absence is 

 in all probability a primary characteristic), but we cannot 

 even find a substitute for them. The monocellular protozoic 

 ancestors of Spongiae possessed, as this is also the case with 

 other Flagellata, contractile vacuoles that served as special 

 emunctory-excretory organelles. No special excretory organs, 

 no nephridia, developed during the transition from the 

 colonies of Flagellata into the polycellular Spongiae. It seems 

 to us that in the Spongiae the formation of special excretory 

 organs was not necessary, because the Spongiae have been 

 able to live easily without them, first, due to the fact that even 

 after the integration their individual cells have preserved a 

 considerable degree of independence; secondly, because their 

 bodies have considerable external as well as internal surfaces; 

 and, thirdly, because the Spongiae have preserved a very 

 slow life rhythm, and because they have generally remained, 

 with a few exceptions only, marine animals. 



On the basis of all this w^e come to the conclusion that the Cni- 

 daria secondarily do not possess any protonephridia because 

 they have adopted the sessile way of life and as such they can 

 dispense with them or they can replace them with other simpler 

 forms. Furthermore, it seems that Dollo's rule is valid for 

 protonephridia as complex excretory organs ; those organs 

 which had completely disappeared cannot be later re-developed. 



