430 THE EVOLUTION OF THE METAZOA 



themselves but rather burrow into the soft floor of the sea. 

 They also show a considerable morphological-physiological 

 specialization in this direction which can be observed above 

 all in the anterior end of their bodies. The trend to preserve a 

 "worm-like form" is clear in the Enteropneusta (there are even 

 species which can be several metres long). Even more impor- 

 tant, however, is the fact that they show a strong inclination to 

 a polymerization which re-emerges in these forms. This poly- 

 merization involves several systems of organs (the sexual, 

 respiratory, digestive, etc., systems) along the third body 

 region which is quite large in these animals. A very important 

 fact also is the circumstance that in the Enteropneusta the 

 maximum number of segments of the perigastrocoele sacks is 

 still preserved. There are still other properties which must be 

 mentioned here such as the presence of a ventral and of a dorsal 

 nerve cord which are equipped with canals, the inclination 

 of these animals to develop an endoskeleton and to form a pair 

 of body folds for the sake of protection , of openings which 

 are used as breathing organs, an inclination of their intestinal 

 tubes to form skeleton-like outgrowths and glands similar to 

 a liver as well as a cavity resembling a pericardium. The ab- 

 sence of metanephridia is conspicuous. 



Thus we come to the conclusion, w^hich has actually been 

 also expressed by many other zoologists, that the recent 

 Enteropneusta must be considered as considerably changed 

 (because they are specialized) descendants from an oligomerous 

 ancestral form, which had been at the same time the starting 

 point of the evolution of the next higher phylum Chordoma, 

 that had developed from this ancestral form during a very 

 remote geologic past. This derivation of the Chordoma from 

 some ancestral form, which they had in common with the 

 recent Enteropneusta, is certainly more probable viewxd from 

 my standpoint than is the other variant w^hich has also been 

 repeatedly suggested by various zoologists and according to 

 which the Chordoma had evolved from an even more primitive 

 ancestor. 



