66 THE MOST PRIMITIVE METAZOA 



animal world from a single unknown stock form, Gastrula or 

 Archigastrula, which was essentially like the gastrula." (Mono- 

 graph on Calcispongiae.) 



These ideas were not accepted even in Haeckel's own time. 

 Thus both Claus and Schmitt disagreed with him, and as Radl 

 (1930) states, " The popular idea of the method of the scientist is 

 that he assembles a series of definite facts upon which he founds his 

 case. We see that this is not always the case. It is not true that the 

 facts which told against the Gastrula were unknown at the time 

 when the theory was propounded, or that the theory was gradually 

 discredited as the facts which contradicted it were gradually 

 accumulated until it finally had to be abandoned. Everything that 

 has ever been cited against the theory was known when the theory 

 was put forward; nevertheless it was widely accepted. Today 

 some still accept it, others do not." Though it is perhaps an over- 

 statement that " everything that has ever been cited against the 

 theory was known when the theory was put forward," Radl's 

 point is made quite clear. The theory was often accepted because 

 it was attractive and not because it was supported by detailed 

 verified factual information. 



One may conclude, therefore, that there is at present no 

 evidence that the sponges arose from an adult gastrula. They have 

 neither a hollow gastrula larva nor are there any simple sponges 

 that are still in the gastrula condition during the adult stage. The 

 situation as Haeckel saw it was based on over-simplification and 

 misinterpretation of the evidence. 



A modification of the Gastrula theory has recently been pro- 

 posed by Jagersten (1955). He suggested that the primitive 

 blastula gave up living and swimming in the sea and started to 

 crawl on the sea bottom (Fig. 23). It modified its structure to 

 become a " Bilateroblastea " with a flattened ventral surface, an 

 arched back, a few sensory cells at the front of the body and the 

 sexual cells inside the body. The centre of the body was hollow 

 and not filled with mesodermal cells. At first the animal fed 

 phagocytically all over the body surface, but as food particles 

 accumulated on the ventral surface the phagocytic ability became 

 restricted to the ventral region. This then became raised from the 

 ground till the animal took up the shape of the " Bilaterogastrea ' 

 as shown in Fig. 23. 



