456 THE EVOLUTION OF THE METAZOA 



Protozoa. The starting point of this evolution were the free 

 living primitive Chordonia which, however, had already lived 

 partly as sessile animals that burrowed slowly in the sea 

 bottom making stops from time to time, just as can still be 

 seen in Awphioxns. The Tunicata evolved in all probability 

 in the sea; they had developed their own larval form which 

 is slightly similar, as this has already been emphasized, to 

 the tertiary larva, to the tetrapodic tadpole. The entirely plank- 

 tonic Larvacea have been the only form which had ever 

 evolved from these larvae; they represent a small side branch 

 whose evolution led no further, just as in the case of the 

 Ctenophora. 



The Ostracodermata represent a blindly ending side branch 

 of fish-like Vertebrata which, however, remained in the 

 benthos and became very specialized; they are now extinct. 

 Because of them the displaced comparison with IJmulus has 

 been attempted. It is very probable that the class Agnatha 

 (here in a very wide sense) w as followed by the Protichtya 

 with their two subclasses, the Acanthodea and the Arthrodira. 

 We know less about the sequence of the Chondrichthya and the 

 Osteichthya, whether they evolved in two parallel lines, or 

 one after the other. 



The phylum Chordonia would thus include nine classes 

 (Fig. 59), the same number as the phylum Oligomeria 

 and the phylum Polymeria (quite "accidentally"); only the 

 first phylum of the Eumetazoa, the Ameria, must be sub- 

 divided into fifteen classes. The question remains open whether 

 all these groups which have been given the rank of classes 

 are really mutually equivalent. Thus, for example, such classes 

 that evolved in all probability by way of neoteny, as such 

 the Ctenophora, Chaetognatha, and the Larvacea, and other 

 classes as the Turbellaria, Mollusca, Polychaeta, Insecta, 

 Echinodermata, Mammalia, etc., to mention a few only. 



The main emphasis, however, must not be placed on the 

 size of each group. A smaller number of subordinate categories, 

 down to the species, can be reached along widely different 



