72 



THE MOST PRIMITIVE METAZOA 



one group, the Dicyemida, represented by Dicyema, is found in the 

 kidney of the Octopus, whilst the other group, the Orthonectidae, 

 represented by Rhopalura, is found inside various marine inverte- 

 brates. Both these groups have a ciliated larva which may bore 

 into a new host and this larva in some ways resembles the 

 miracidium larva of the digenetic trematodes (Figs. 25 and 27). 

 The Mesozoa also have a complex life cycle, and this together with 

 the larval structure has led writers such as Stunkard (1954) and 

 Caullery (1951) to think that the Mesozoa are probably digenetic 

 trematodes. The recent Traite de Zoologie edited by Grasse also 

 places the Mesozoa amongst the platyhelminthes. We thus have 

 two views concerning the Mesozoa; one that they are primitive 

 animals, the other that they are degenerate parasites. 



-INVERTEBRATE HOST- 



Plosmodium >-Agannete- 



Male 



Female 



•Ciliated larva-*- 



- Zygote 



Fig. 26. Diagram of the life cycle of Rhopalura. (After Caullery.) 



It should be stated at the very beginning of this discussion that 

 the Mesozoa have suffered as a group in that various non-related 

 animals such as Haplozoon have been thrust into the Mesozoa 

 though in fact their affinities are elsewhere (see p. 41). There is 

 also some doubt about the closeness of the relationship between the 

 Orthonectids and the Dicyemids. The resemblance lies in their 

 simple morphology and the fact that both have a ciliated larva, 

 but since they are both internal parasites, the simple morphology 

 is suspect straight away. One knows that other internal parasites 

 such as the males of Bonelia, or the parasitic cirripedes, become 

 very simplified. On the other hand the adult trematodes and 

 cestodes are not morphologically simple — there being a tremendous 

 development of the reproductive systems. 



Dodson (1956) has suggested that though a parasitic life can lead 

 to morphological simplifications in the parasite, it does not 



