THE MOST PRIMITIVE METAZOA 



89 



Tetraplatia Hydroctena 



Fig. 31. Aberrant coelenterates. 



Tetraplatia shows superficial resemblance to the Miiller's larva of 

 the polyclad platyhelminthes. (After Krumbach.) 



Hydroctena. This medusoid form shows certain resemblances to 



a ctenophore. (After Dawydoff.) 



Relationship of the Ctenophora to the Turbellaria 



Although typical ctenophorans such as Plenrobrachia or 

 Hormiphora are round pelagic animals there are some creeping 

 forms. It was these creeping forms and in particular Coeloplana 

 (Fig. 30) that led Lang (1884) to suggest that the ctenophores gave 

 rise to the polyclad Turbellaria. It is not hard to bridge the 

 gap between the pelagic forms such as Pleurobrachia and creeping 

 forms like Coeloplana. Thus Lampetia is a semi-globular form that 

 sometimes crawls on its everted pharynx. Ctenoplana in its 

 swimming form is clearly a ctenophore (Fig. 30B) but in its 

 crawling form it spreads itself out on its oral lobe and becomes a 

 flat animal. Finally Coeloplana is a flattened form like a turbellarian 

 (Komai 1922) and it looks very much like a link between the 

 ctenophores and the polyclad Turbellaria. In transverse section 

 Coeloplana has a complex structure (Fig. 33) and it is not sur- 

 prising that Lang thought that it was the forerunner of the 

 turbellarians. In particular he associated it with the polyclads 

 because of the many branches of the gut. The polyclad resem- 

 blances can be seen from the following list. 



