14 



flat plate or disc formed of two layers of cells, the upper 

 epiblastic and the lower hypoblastic — being regarded as the 

 earliest common ancestor of the Metazoa. The Placula is 



Fig. 9. 



Planula stage in the development of a Metazoon ; A, planulawith solid hypo- 

 blast; B, planulawith a cavity in the hypoblast. 



certainly a form which might be very naturally assumed by a 

 small colony of unicellular organisms, and Butschli has 

 shown how it might readily be modified during its formation 

 so as to assume either a Planula or a Gastrula structure. 



From the early Metazoon represented by one of these, 

 or some closely related embryonic form (the point marked 

 Gastrea in the table), several lines must have diverged. 



Fig. 10. 



Placula— a hypothetical ancestral form (after Butschli). 

 development are shewn. 



Two stages in its 



One of these, on the right, slopes considerably downwards to 

 end in the Dicyemida and the Orthonectida. These two 

 lowly organised groups are parasitic, and probably degene- 

 rate. They agree with the Metazoa in being multicellular, 

 but differ from them in having the endoderm represented by 

 a single cell only, and they have some peculiarities in regard 



