PREVIOUS INTERPRETATIONS OF CNIDARIA 



113 



The layer of the jelly-like substance partly takes over the 

 irole of an extensible internal skeleton as early as in Anthozoa, 

 and even more so in the Scyphozoa and in the Hydrozoa, con- 

 cordantly with its new position: the appendices of the muscular 

 tissue of the two epitheUa, which find support in this skeleton, 



Fig. 21. Half schematic longitudinal section of the body wall of 

 Hydra; left, the skin, right, the gut; lentre, the mesohyl (after Hadzi). 



passively share the stretching and the shrinking which takes 

 place during the thickening and the flattening of the body. 

 This is true for the actively moving parts of polyps. In medusae 

 this layer of a jelly-like substance can occasionally be of con- 

 siderable size; here it represents a firm elastic substance which 

 takes over the role of an antagonist to the rhythmically contract- 

 ing subumbrellar muscles. In the Anthozoa and in the Scypho- 

 zoa this layer of a jelly-like substance contains individual, 

 usually amoeboid, mesohyl cells, while in the Hydromedusae 



