PREVIOUS INTER PRETATIONS OF CXIDARIA 197 



true for other Anthozoa either. Why should we deny the 

 mesenchymal (mesodermal) character to such cells that onto- 

 genetically and phylogenetically arise in the skin layer and 

 which later migrate into the middle layer, when in other 

 animal types the same cells are considered forthwith as mesen- 

 chymal cells? 



As for the cellular structure of the skin laver it can be stated 

 that from our standpoint we can expect in Anthozoa the con- 

 ditions to be more primitive than in Hydrozoa (we speak here 

 of the polyp generation only). Similarly as in TurbeUaria, where 

 a sHghtly cellularized epidermal epithelium (ectocyte) can re- 

 gularly be found in the primitive Acoela and where we there- 

 fore speak of a syncytial structure, we find cases with a plasmo- 

 dial (syncytial) ectoderm (epidermis) first in Anthozoa among 

 Cnidaria. The cellularization, i.e. the morphological-physio- 

 logical division of the elementary units of Eumetazoa (of the 

 true cells) in the primarily polycaryontic plasmodium becomes 

 more and more complete in the direction Anthozoa -> Scvpho- 

 2oa -> Hydrozoa. This, however, is not true for the skin laver 

 only. In sessile animals we find this cellularization to be con- 

 nected with an epitheUzation, and it is especiallv developed 

 in those animal types that form cormi. In this sense the 

 middle layer (and the ner\^ous system) lags behind. In 

 Cnidaria the middle layer retrogresses more and more until 

 finally the climax of this development is reached in hvdro- 

 medusae and in the hydranths of the corm-forming Hyd- 

 roidea. I have already stated that this development has never 

 led to a complete loss of the middle layer (it can occur locally 

 only). 



Finally we also find the actual conditions of the skin gland 

 cells to agree better with our interpretation of the origin of 

 Cnidaria than with the earUer theory. A mucous membrane 

 can be found in the primarily soHtary Anthozoa only. Some 

 Actiniae (e.g. the common i&d Actinia equina) are able to Uve 

 in the tidal zone because at ebb-tide they cover their bare and 

 contracted bodies with a thick layer of mucus. This lowest 



14 



