THE MOST PRIMITIVE METAZOA 



73 



(a) Infusoriform 

 larva 



(b) Older larva 



Fig. 27. Mesozoan structure. Dicyema. 



(a) From Hyman after Nouvel. 



(b) From Hyman after Lameere. 



(c) From Hyman. 









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mz 



4 • 



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(c) Nematogen 



necessarily do so. Thus animals such as the leech are parasitic 

 but complex in structure, and, as we have already mentioned, 

 many of the platyhelminthes and nematodes are quite complex. 

 When one also considers the fact that the mesozoans have a 

 complex life cycle, some stages of which are not yet known, it 

 would appear premature to place the mesozoans in a key position 

 between the Protozoa and the Metazoa. 



It is possible to draw up lists of the simple characters and the 

 platyhelminth-like characters of the Mesozoa. These are as 

 follows. 



Simple characters of the Mesozoa (non-platyhelminth 

 characters) 



(1) They are multicellular animals with no differentiation into 

 endoderm, ectoderm or mesoderm. 



