PREVIOUS INTERPRETATIONS OF CNIDARIA 121 



Stratum") one longitudinally and one transversally placed stra- 

 tum, and similarly in the basal zone. 



The hydrosome of some Siphonophora (Apolemia uvaria^ 

 according to K. C. Schneider, and Halistemma tergestimim^ accord- 

 ing to Claus) can be mentioned as an example of newly formed 

 muscle fibres. This so-called hydrosome of Siphonophora is 

 nothing else but the hydrocaulus of the Hydroidea which has de- 

 veloped no muscle fibres because the slight general contractility 

 of the cytoplasm has been clearly sufficient, and because of the 

 periderm which as an external skeleton covers the "coenosarc." 

 Yet a strong contractility of the "stem," as the bearer of the 

 whole colony, has been useful for the planktonic Siphonophora 

 and we can therefore observe here not only muscle fibres that 

 have been developed as appendices to the skin layer, but also 

 that the epithelium muscle cells which had been developed in 

 this way have sunk into the "mesogloea." It is natural that 

 K.C. Schneider speaks here of "... in die Tiefe gesunkenen 

 Deckmuskelzellen" which must be interpreted "also als eine 

 Vorstufe echter Muskelzellen" (K.C. Schneider, 1902:607). 

 According to our interpretation we have here, not muscle cells 

 in the process of development, but rather parts of the so-called 

 Deckmuskelzellen which have been secondarily developed into 

 the former. 



The subepithelial cells are frequently mentioned in the histo- 

 logy of Cnidaria. They lie close-wedged between the basal 

 epithelium cells and they are usually attributed to the skin 

 layer or to the ectoderm. This, however, is wrong. Every- 

 thing that lies under the skin epithelium, whether migrated 

 to this place from the epithelium through immersion, or that 

 originated here, belongs to the middle layer ("mesoderm"), 

 and it must be attributed to it even if there is no sharply de- 

 lineated and clear limit in between. Cnidocytes can serve here 

 as an example; they are recruited from indifferent "mesen- 

 chymal cells," and, after their further development, they are 

 ranged into the skin epithelium either "on the spot" or after 

 a shorter or longer wandering; they must therefore be 



