PREVIOUS INTERPRETATIONS OF CNIDARIA 115 



follows Dollo's rule and it does not represent a return to those 

 conditions that had been observed in the movements of their 

 creeping ancestors. 



We will maintain, even if this may appear surprising, that 

 it is right to attribute the muscular tissue of Cnidaria to the 

 middle layer even in those cases where it appears as a basal 

 appendix of the epithelium cells of the ecto- or entoderm and 

 where it does not possess its own nuclei, i.e. where it does 

 not consist of independent cells. In numerous anthopolyps we 

 can observe how this muscular tissue subsequently develops 

 locally into independent cells with their own nuclei. Chester 

 (1912:755) observed in the species V seudoplexaura crassa how the 

 epithelial cells (they were classified as entodermal cells) that 

 occur in mesenteries and that had each previously produced 

 a single differentiated muscle fibre, leave the epithelial group 

 and sink into the subepithelium, i.e. into the intermediate 

 layer, and in this way develop into the pure muscular cells. 



We can obtain a good understanding of the muscular con- 

 ditions in Cnidaria only if we derive them from conditions 

 that can be observed in TurbeUaria, and if we pass from 

 the situation that can be observed in Anthozoa to those that 

 occur in the Scyphozoa and in the Hydrozoa. A very mislead- 

 ing idea of the primitive nature of the so-called neuro-muscu- 

 lar-epithelial cells has been proposed by Nikolaus Kleinenberg. 

 According to this interpretation such a cell has differentiated 

 in three directions, into the pure epithelium cells on the surface, 

 and into the muscle and nerve cells in the interior of the body. 

 With regard to the nerve ceUs it soon became necessary to 

 relinquish this concept after the nerve cells had been discover- 

 ed in Hydra. Yet in spite of this, the idea of the primitive 

 character of the muscle epithelium cells has been preserved 

 down to the present day. H. Wolf (1903) developed this 

 hypothesis for the nerve cells and supported it with beautiful, 

 even if completely imaginary, pictures to show that they ori- 

 ginate, like the muscle ceUs, in the immersed epithelium cells. 

 It has seemed that this interpretation was supported by the 



