PREVIOUS INTERPRETATIONS OF CNIDARIA 137 



Parallel to the development of their muscle system and of 

 their sense organs, which is connected with an increased den- 

 sity of the nervous tissue, we can observe a sinking of the 

 latter into the skin epithelium, thus into the jelly-like basic 

 substance of the intermediate layer. This development can be 

 observed along the margin of their umbrella and also in their 

 tentacles, as has been proved in Pelagia by Krasiriska (1914). 

 In principle, it would be wrong to suppose the presence of 

 such an advanced nervous system in the sessile polyps. 



The 'Efnunctory-Excretory Organ 



As is well known, no visible special excretory organ of a type 

 which would resemble a protonephridium, for instance, can 

 be found in the Cnidaria. On the other hand, we can often find 

 a well developed protonephridial excretory system in the 

 Turbellaria. Those who adhere to the old interpretation, find 

 an easy explanation of this difference: they suppose that in 

 Cnidaria the absence of protonephridia is a primary charac- 

 teristic, because the Cnidaria are, so they believe, as far as 

 their primary organization is concerned, at a lower level of 

 evolution. Yet, we too can find an explanation of this diffe- 

 rence between Cnidaria and Turbellaria, which at the first 

 glance seems to be so important, on the basis of the following 

 arguments: 



(1) There is a considerable uncertainty about the origin of 

 protonephridia that have developed later into other nephridial 

 organs, above all into the metanephridia of molluscs among 

 the Ameria and Polymeria. While there are no protonephridia 

 in "Coelenterata," they appear, as it seems, suddenly in the 

 lowest "Coelomata," in the Platyhelminthes. They have, with- 

 out any previous intermediate stage, a well-developed tube 

 system with specialized terminal cells equipped with beating 

 undulipodia; internally they end blindly, while externally they 

 end in pores. It cannot be supposed that they have developed 

 as such out of nothing. Krumbach (in his Handbuch der Zoo- 



