CNIDARIA 125 



jsume, then, that Scyphomedusse and Anthozoa are de- 



jended from a common polypoid ancestral form which was 



already characterized by the possession of an ectodermal 



oesophagus, still the direct union of the two groaps does not 



seem to be as yet sufficiently established. 



General Considerations on the Cnidaria. — The Cni- 

 daria constitute a very homogeneous, well-defined branch of 

 the animal kingdom. We assume that the fundamental and 

 ancestral form from which they are derived was a polyp 

 similar to Hydra, the chief axis of which was the same as 

 in the preceding free-swimming ancestral form. A free 

 oral pole and a pole of attachment can be distinguished. 

 The latter corresponds to the anterior pole of the free- 

 swimming ancestral form. The radial type in the structure 

 of the Cnidaria has arisen in connection with the attached 

 mode of life, whereas in many Cnidaria, as the result of 

 stock-formation, a bilaterally symmetrical type is second- 

 arily developed. It appears that even the ancestral form of 

 the Cnidaria had developed the quadriradial structure, so 

 that those forms in which, on account of the arrangement 

 of the tentacles, no definite secondary axes can be recognized, 

 like the Corynidce and Clavidcp., would represent a secondary 

 modification. The growth of the Cnidaria frequently takes 

 place by the typical intercalation of new radii between those 

 already present (Hatschek) . 



The Hydrozoa are derived directly from this Hydra-like 

 ancestral form (Archihydra), whereas the common ancestral 

 form of the Anthozoa and Acraspeda is developed from it 

 by the formation of an ectodermal oesophagus and radial 

 septa. The presence of the longitudinal muscles in these 

 septa indicates that they were developed in connection with 

 the attached mode of life. In the ontogenetic series, it is 

 true, the septa often make their appearance before the 

 attachment and before the development of the tentacles, 

 from which Goette concluded that there was an ancestral 

 form, called a Scyphula, common to the Anthozoa, Acra- 

 speda, and Ctenophora, which was characterized by a free- 

 swimming mode of locomotion and by the possession of an 

 oesophagus and radial septa. It is possible, however, that 



