THE MOST PRIMITIVE METAZOA 



63 



(o) Monerula 



(b) Cytula 



(c) Morula 



(d) Blastuta 



(e) Gostrula 



Fig. 21. Haeckel's concept of levels of organisation. He suggested 

 that animals evolved through the successive adult stages shown in the 



above figure. 



some stage of their embryonic development and that a group of 

 animals existed which were adults but which were still at the 

 gastrula stage. Such animals were not known, but later Haeckel 

 thought he discovered such an adult group of animals and 

 he called them the Physemaria, an example of which was 

 Haliphysema. 



Haliphysema, according to Haeckel, had the structure of a little 

 vase. The walls of this vase were made up of two layers of cells ; 

 the inner layer was flagellated, the cells having a collar-like 

 structure. The outer layer was made up from a syncytium of 

 cells. These outer cells took up stones and spines from the environ- 

 ment and covered the animal with a protective layer. They were 

 adults, since Haeckel described a series of gonadial cells which 

 were formed from the endoderm and then became liberated into 



