62 the most primitive metazoa 



Origin from the Gastrula 



This view holds that the sponges evolved from some form of 

 gastrula and was propounded mainly by Ernst Haeckel. Haeckel 

 from his studies of the embryology of the sponges (1872) decided 

 that the larval form of the sponge was a gastrula larva. He also 

 thought that certain other animals such as Haliphysema were 

 primitive sponges, though he later changed his mind. 



Haeckel's views have had considerable influence on our current 

 zoological concepts. It was Haeckel who devised many of our 

 current words such as Phylum, Blastula, Morula, Gastrula, 

 Ontogeny, Phylogeny and many more. It is perhaps relevant that 

 we should spend a little time reporting how Haeckel developed his 

 ideas and concepts and the way in which these fitted in with 

 views on the origin of the sponges. 



Haeckel spent many years developing his views on the phylogeny 

 of the animal kingdom. In effect he studied both the structure 

 and embryology of each of the various groups. Then by marrying 

 the facts and ideas from comparative anatomy and embryology, 

 Haeckel developed a sweeping plan of the relationship and evolu- 

 tion of various animal groups. He supported his schemes with 

 detailed arguments and when his opponents failed to understand 

 his arguments Haeckel devastated them with a barrage of crushing 

 sarcasm against their misinterpretation of his own specialised 

 terminology. 



Simply stated, Haeckel's view was that the most primitive 

 animal was a small non-nucleated mass called the Monerula (Fig. 

 21). This was followed by a later group of animals that had a 

 nucleus and this state was called the Cytula. The Protozoa are 

 at the cytula stage. The next group of animals formed a solid 

 mass of nucleated cells called the Morula. The Morula led to a 

 more complex form that had a hollow centre and a single layer of 

 cells, the Blastula. Certain cells such as Volvox are almost at the 

 Blastula stage. The next stage in evolution was the Gastrula stage, 

 in which the animal had a double wall, a ciliated exterior and a 

 hollow gut. 



The Gastrula assumed tremendous importance in Haeckel's 

 phylogenetic speculations. He thought that the Gastrula was the 

 ancestor of all the Metazoa, that it occurred in all the Metazoa at 



