334 THE EVOLUTION OF THE METAZOA 



"theinversionof germ layers" which is so typical of Spongiae 

 and which has been the reason that the name Enantiozoa has 

 been given to them, is simply a "multipolar inwandering." In 

 reality it appears also as an invagination and as a unipolar in- 

 wandering, thus in all the forms that we are used to find in a 

 normal "gastrulation," yet it is usually reversely polarized! 

 In the same way we could speak about germ layers in Metaphy- 

 ta, or at least of something that is similar to this ontogenetic 

 process. We are therefore not yet justified in supposing that 

 there has been a common origin. Yet in spite of this, all the 

 organisms represent a community, even if the complex organ- 

 isms evolved several times. We must not forget that all the 

 metabionts had originally evolved from the primarily solitary 

 Flagellata. It is therefore not surprising that we find many 

 common elements in them in spite of the fact that they 

 evolved several times and invarious ways from these Flagellata. 

 There is no species of Spongiae— at least as far as I know— 

 which begins its ontogeny with a division of karyons, thus 

 without a simultaneous division of cells or without a cleavage. 

 In the Eumetazoa on the other hand w^e can find such a 

 development quite frequently even in such groups as the 

 insects. We can rightly consider this type of development 

 as a result of a secondary evolution. The larva (spongula) 

 of Mycale passes subsequently, according to H. V. Wilson 

 (1935), into a syncytial state. A formation of syncytia, how- 

 ever, can also be found elsewhere among the Spongiae, 

 especially in those cases where we have a formation of macro - 

 scleres. We are therefore not allowed to conclude immediately 

 that in all those cases there had been a polykaryonic phylo- 

 genetic origin where an animal type of a metaphyte can be 

 found w^hich forms syncytia. The well-kno\\'n case can be 

 mentioned here in passing where under the influence of a 

 very simple saline solution a larva of Polychaeta was changed 

 into a plasmodial animal (Lilly). It w^ould be wrong to con- 

 clude on this basis alone that all the Metazoa had evolved 

 from some plasmodial ancestors. 



