THE MOST PRIMITIVE METAZOA 



OCTOPUS- 



75 



Rhombogen 

 1 



occasional 



^. Secondary 

 Nematogen 



Stem Nematogen 



???+ 



Infusorigen 



[Infuso r ifo"m larva] 



Fig. 28. Diagram of the life cycle of Dicyema. (After 



A IcConnaughey.) 



Caullery (1951) stated that it was probable that the adult of 

 Orthonectids such as Rhopalura would on future examination 

 show greater histological differentiation. ' A more careful 

 histological analysis than so far made will probably disclose a 

 nerve ring. What is lacking is a digestive apparatus, as in the 

 Monstrillidae, and here this is almost certainly because the life 

 of the adult is here even more ephemeral, and entirely devoted to 

 the production and dissemination of larvae." Caullery is clearly 

 of the opinion that the Orthonectids are degenerate forms. 



There is no certainty that the Orthonectids and the Dicyemids 

 are as closely related as their grouping together in the Mesozoa 

 suggests. The resemblances are mainly that both have a ciliated 

 larva and the adult structure is multicellular without a gut or 

 organ systems. The life cycle of Dicyema is not yet fully known 

 and so it is difficult to compare it with Rhopalura. There is a 

 plasmodial stage in Rhopalura which has not been described for 

 Dicyema. The ciliated larva is not identical in structure in the two 

 forms. 



If the Mesozoa are not primitive, they can be considered as 

 degenerate digenetic trematodes, possibly the miracidium larva 

 of some trematode that has become the end stage of development, 

 e.g. the Mesozoa are neotenous miracidia. There are, as we have 

 seen, certain resemblances between the miracidium and the ciliated 



