THE SERIAL RELATIONS OF INVERTEBRATA. 



579 



of the Gregarinidce^ Yampyrella of the Heliozoa, Protomo- 

 7ias of the Flagellata. A Moneran, ciliated over its whole 

 surface, which might stand in the same relation to the Ojm- 

 linina, Catallacta, Tentaculifera, Ciliata^ is at present un- 

 known. The Protozoa thus fall into the following series : 



Peotozoa. 



T. II. III. 



Protogenes. Protamoeba. Myxastrum. 



IV. 



Vampyrella, 



Foraminifera. Protoplasta. Gregarinidce. 



Heliozoa, 



I 



Radiolaria. 



V. 



I 



Tent aculif era. 



VI. 

 ? 



Catallaeta, 



I 



Ojxdinina. 



VII. 



Protomoiias, 



Flagellata, 



Ciliata. 



I am unable to trace any one of these series of modifica- 

 tions further; that is to say, to find forms which actually 

 bridge over the interval between any one of them and the 

 Metazoa^ though it is easy enough to imagine what such forms 

 might be. The spheroidal free-swimming monad aggregates, 

 such as TTvella and Polytoma^ and 3Iagosphciera itself, are, 

 in many respects, comparable to Physemarian or Poriferan 

 embryos ; while an animal Volvox would be a sort of perma- 

 nent vesicular morula. So, one of the higher Infusoria^ if it 

 became multinucleate, like an Opalina^ would approach the 

 lowest Turhellaria. 



The axial cell of a I>lcyeina^ from the protoplasm of which 

 its ciliated and nonciliated germs are produced, is, to a cer- 

 tain extent, comparable to the capsule of a Radiolarian ; 

 while, on the other hand, a Radiolarian with a multinucle- 

 ate cortical layer would approach the structure of Dicyema. 

 And if what is at present known of Dicyema gives a just 

 conception of the essential points of its entire history, it un- 

 doubtedly, as E. van Beneden has suggested, represents a 

 type intermediate between the Protozoa and the Metazoa, 



