PREVIOUS INTERPRETATIONS OF CNIDARIA 203 



localized place of origin of cnidocytes, combined with an 

 increasingly complicated migration of half -formed cnidocytes 

 to those parts of the animal body where they can be used 

 (this has first been described by Hadzi (1907), later confirmed 

 by numerous other authors). From a physiological and ecologi- 

 cal standpoint we can notice a further specialization into penet- 

 rating, volvent, and glutinating cnidae, a development which 

 again has reached its climax in Hydrozoa. 



Finally we can observe that when the climax has been reach- 

 ed in the progressive evolution the subsequent development 

 can turn in a retrogressive direction. This can be seen, first, 

 in the fact that a polycnidom can secondarily be changed 

 into a monocnidom, thus to a simplification in the selection 

 of cnidae, as this is the case with Trachylinae. Another retro- 

 gression can be observed in those cases where cnidae degene- 

 rate, as can be seen in the so-called rhopalonemes and desmo- 

 nemes in Siphonophora (Diphyes). Cnidae have lost their 

 primary function in numerous hydromedusae, especially in 

 Trachylinae. Here they remain, half developed, in the middle 

 layer; amassed in the margin of the umbrella and behaving 

 passively, they serve as an elastic and strengthening inner 

 skeleton. 



As a conclusion it can be stated that cnidoid forms belong 

 to the category of non-living, even if organic, secretions of 

 the cytoplasm. They first appear, in an acellular state, as early 

 as in Protozoa; they have been continuously progressively 

 developed in several directions and they have reached their 

 climax in Dinoflagellata (Kofoid and Swezy, 1921; Weil, 1925; 

 Chatton, 1914); they have reached another climax in Infusoria 

 (Kriiger, 1934). The Turbellaria, as the first Eumetazoa, had in- 

 herited their inclination to form cnidoids and passed it on to 

 several of their descendants. An absolute climax has been 

 reached in this development in the sessile Cnidaria. The 

 progressive line of evolution points clearly in the direction 

 Anthozoa -> Scyphozoa -> Hydrozoa, reaching its final and 

 general culmination in Hydrozoa. In the latter, a partly retro- 



