72 THE EVOLUTION OF THE METAZOA 



trends between the three classes of Cnidaria. It can be 

 generally stated that the farther away from Antho5:oa we move 

 in the direction of Scyphozoa and Hydrozoa, the fewer pro- 

 perties of Turbellaria we meet; all this is due to a divergent 

 development and to the prevalence of new properties and 

 peculiarities which are typical of Cnidaria. Yet while we can 

 observe that there are also some side branches which have 

 developed out of Turbellaria into several new directions, either 

 in the direction of animal types which have adopted a parasitic 

 way of life (Trematoda, Cestoda), or such that have adopted 

 the free mobility and the free way of life (Nemertinea, etc.; 

 most probably after the transition to an euproctous state, also 

 in the direction of Mollusca), nothing similar can be observed 

 in Cnidaria. On the basis of all this we can conclude— and 

 rightly I think— that these Cnidaria represent a blind side- 

 branch on the tree of Ameria. We can see the cHmax of a 

 partly progressive development in the strongly individualized 

 cormi of Hydrozoa, in the Siphonophora. It is possible, though 

 it is difficult to prove, that Cnidosporidia, which are attributed 

 to Protozoa, had possibly been developed from the parasitic 

 Narcomedusae. This hypothesis seems to be supported by the 

 so-called polar capsulae, thus cnidae, and by the polycaryontic 

 (polyenergetid) state of their vegetative phase connected with 

 the phenomenon of cellulation. 



Parasites have occasionally been used — and this success- 

 fully—to explain the phylogenetic conditions of their host 

 animals. This can be considered to be a special method in 

 phylogenetics. If this is valid for parasites then its validity 

 could also be extended to symbionts. It is probably not a mere 

 chance that a frequent symbiosis of monocellular algae that 

 live intracellularly (e.g. Zoochlorella and Zooxanthella) has been 

 observed on one hand in certain Infusoria, and on the other in 

 the Turbellaria and Cnidaria. In the two last mentioned cases 

 this symbiosis can be so strong that it becomes strictly oblig- 

 atory and that in connection with this, systems have been 

 developed which secure the "inheritance" of their symbionts. 



