PHYLUM PLATHELMINTHES 



107 



ture, nervous system, and reproductive organs which have their 

 origin as purely epitheUal structures in the lower coelenterates 

 become associated with the mesoglea in the higher coelenterates 

 and transform this layer into a true mesoderm in the Ctenophora. 

 This trend of development and specialization of the mesoderm 

 is carried still further in the highly organized parenchymatous 

 body of the Plathclminthes. 



Fig. 59. — ^4. Coeloplana adult in dorsal view. B. Larva of Coeloplana directly- 

 after hatching. {Redrawn from Komai) . 



There are some organisms which show striking combinations 

 of flatworm and ctenophore characteristics. Coeloplana and 

 Ctenoplana are two such genera to which unparalleled phylo- 

 genetic significance has been attached. Coeloplana in its general 

 form (Fig. 59 A) resembles a flatworm, but on its dorsal surface 

 it bears a sense organ like the one found in similar position on 

 the ctenophores. The dorsal tentacles and the tentacular sacs 



