THE MOST PRIMITIVE METAZOA 87 



Scyphozoa. For the ctenophores in many ways resemble the 

 scyphozoans; thus, both have a poor regenerative ability in the 

 adult, they both develop thick mesenchyme and they both 

 develop muscles. Considerable interest was aroused by the dis- 

 covery of the medusa Hydroctena. Haeckel suggested that it was 

 an ancestral form to the Ctenophora and it certainly shows a 

 superficial resemblance to a ctenophore as can be seen from Fig. 31. 

 The resemblances are due to the ovoid shape, the two tentacles 

 (which Haeckel thought could be retracted into pockets at their 

 base) and the gut being divided into four pockets. On the other 

 hand there are many differences. Hydroctena has a circular canal 

 around the perimeter of its body, the tentacles are oral and non- 

 retractile (those of the ctenophores are aboral and retractile). 

 There is no statocyst, it has nematocysts and not lasso cells, and 

 the gonads develop on the wall of the manubrium instead of the 

 radial canals. What is of interest here is the manner in which the 

 ctenophoran form can be imitated by a medusoid form. 



Another interesting coelenterate that shows certain ctenophore- 

 turbellarian affinities is Tetraplatia. It was classified as a nar- 

 comedusan hydrozoan by Carlgren (1926) placed in a separate 

 order of the Hydrozoa, the Pteromedusae by Hand (1955), but 

 identified as a coronate scyphozoan by Krumbach (1927). Its 

 external form is elongate and very much like that of a Muller's 

 larva. There are eight lappets around the body, this giving some 

 resemblance to a ctenophore; on the other hand the mouth is 

 terminal (Fig. 31). Tetraplatia' s affinities are further discussed by 

 Ralph (1959). 



It is unfortunate that the planula larva is not more common in 

 the ctenophores. The only known case is in Gastrodes, which is 

 parasitic on Salpa. Otherwise the ctenophores have a typical 

 cydippe larva. There are some clear affinities between the 

 coelenterates and the ctenophores but just how closely the two are 

 related is hard to say. If further investigations show the presence 

 of nematocysts to be more widespread than just in Euchlora and 

 if they are of a similar pattern to the coelenterate nematocysts and 

 not like those of the protozoan nematocysts, it will indicate that 

 the ctenophores can be included in the Cnidaria. The relationship 

 between the Hydrozoa, Scyphozoa and Anthozoa seems to be a 

 closer one than that of these three to the Ctenophora. 



