INCERTyE SEDES — MESOZOA. 



123 



sluggish constitution. It does not seem likely that they 

 have led on to anything higher, they rather represent a 

 by-road in Metazoan evolution. 



Incert^e Sedes. Mesozoa. 



The title Mesozoa was applied by Van Beneden to some very simple 

 organisms which appear to occupy a very humble position in the 



Fig. 53. — A. young Dicyema. — 

 After Whitman. B. Female 

 Orthonectid (Rhopalura 



Giardii). — After Julin. 



e., Ectoderm; en., inner endoderm cell 

 with nucleus («.) ; and embryo (em.). 

 Note the segmentation and the 

 fibrillation supposed to be muscular. 



Fig. 54. — Salinella. — 

 After Frenzel. 



1. Longitudinal section — 



a., anterior; p., pos- 

 terior. 



2. Transverse section. 



Metazoan series. He regarded them as intermediate between Protozoa 

 and Metazoa ; but others have remarked on their resemblance to 

 Platyhelminthes, and especially to the sporocysts of certain Flukes. 

 They may perhaps be regarded as precociously reproductive sporocysts. 

 It will be enough here merely to notice four types : — 



