THE MOST PRIMITIVE METAZOA 



67 



(b) 



Fig. 23. Bilaterogastrea theory. Jagersten has suggested that 

 the primitive larval form was that of a bilaterogastrea. The 

 planula larva (a) settled on the ground, raised its ventral surface 

 to accommodate food material (b, c) and so developed a gut (d, e, 

 f). It then takes up the form of a Bilaterogastrea. (From 



Jagersten.) 



Jagersten's scheme is derived from his view that the primitive 

 form had a hollow centre and was not filled with mesenchyme. 

 The reason he thinks that the primitive form had a hollow interior 

 is that otherwise the various groups of animals such as the 

 coelenterates, platyhelminthes, etc., would have had to develop a 

 hollow gut independently and on several different occasions. 

 Secondly, though he agrees with the statements that the endoderm 



6— IOE 



