THE NEW GENEALOGICAL TREE 367 



that it is a result of a secondary simplification (so-caJled "de- 

 generation") because these animals are very speciaUzed endo- 

 parasites which pass through a phase, even if short, of a free 

 life. This, however, does not mean that there had been no 

 partially progressive evolution, especially at the point of the 

 development cycle; this can be observed in the Sporozoa 

 among the Protozoa and in the Platyhelminthes (Trematoda). 

 We will leave it undecided whether this was a case of the 

 neoteny with the miracidium larva as the initial form, or 

 something else. 



This interpretation will appear a priori more probable to 

 those who accept the theory that the Metazoa evolved from 

 some colonies if they take into consideration the classical 

 interpretation that evolution proceeded along the line blas- 

 taea-gastraea. Yet the planula thesis too seems to be now 

 much less firmly rooted (Hatschek suggested the phylum 

 Planularia, with the Planulozoa as the only cladus). 



Those who accept the polykaryonic theory could derive the 

 Orthonectidea directly from the CiHata by way of a poly- 

 merization and of a partial cellularization which can be easily 

 identified. There is, however, a large number of facts which 

 completely disprove such a derivation. The equally numerous 

 similarities appear as analogies, as a consequence of a retrogres- 

 sive development which was caused by their endoparasitism. 

 The Dicyemida seem to be much more reduced forms than the 

 Orthonectida because they are built of a smaller and definite 

 number of skin cells. It cannot be therefore expected (as this 

 has been repeatedly emphasized) that anything essentially new 

 could come to light with a discovery of those parts of the life 

 cycle of these animals which have remained so far unknown. 

 In all probability such a discovery will not help us to get a 

 more precise knowledge of the origin of the Orthonectoidea, 

 even if such a possibility cannot be completely excluded. 



We come therefore to the conclusion that there are no 

 Mesozoa in the sense as this word has been used till now. 

 Neither had they ever existed, because both the Spongiae and 



