PREVIOUS INTERPRETATIONS OF CNIDARIA 141 



The following factors had supposedly contributed to the com- 

 plete disappearance of special emunctory-excretory organs: (a) 

 life in the sea; (b) the abandonment of active movement; (c) 

 the progressive diminution of the size of polyps in connection 

 with the formation of cormi; (d) the relative enlargement of 

 free surfaces (external as well as internal); and, (e) the epitheU- 

 zation of the tissue. 



It is characteristic that during the subsequent evolution of 

 the Cnidaria, protonephridia have never reappeared, either in 

 the secondarily solitary polyps whose size has also been 

 simultaneously increased, or in the secondarily free medusae 

 that have also reached a considerable size. In all probability 

 this can be attributed to Dollo's rule of irreversibility. The 

 functions of the abandoned organs have been taken over by 

 the parts of the body that have been preserved, occasionally 

 with a partially progressive development of the latter (the 

 pores in numerous medusae, the so-called liver in Chondro- 

 phores among Siphonophora). 



(3) We can see, if we look around at a somewhat broader 

 circle, especially among the numerous subtypes of Ameria, 

 how frequently under special conditions protonephridia have 

 been reduced or even abandoned. Protonephridia are extremely 

 reduced in the Endoprocta (Kamptozoa, according to Cori) 

 that live mostly in the sea and that have become sessile 

 animals. Two very short canals, each with one ciHated cell 

 at its internal end and with one pore, is all that has been 

 preserved of all the variously twisted canals with their numer- 

 ous terminal organs. The protonephridia have completely 

 disappeared in the species of two orders, out of the three,, 

 of endoparasitic Acanthocephala. In a majority of Nemertinea 

 that live in the sea, the nephridial apparatus, which is other- 

 wise considerably specialized, is strongly reduced, and it is 

 completely abandoned in numerous bathypelagic species. 



Most interesting in this connection are the Ctenophora,. 

 Zoologists are usually of the opinion that the Ctenophora 

 as Coelenterata, do not primarily possess any protonephridia. 



