PREVIOUS INTERPRETATIONS OF CNIDARIA 133 



organs. In spite of this, however, there are differences between 

 the two medusoid forms, even if we disregard such differences 

 as, for example, the presence of a velum (only in Hydromedu- 

 sae), or its absence (primary in all Scyphomedusae, secondary, 

 and rare, in Hydromedusae, e.g. in Obelia species), and the 

 completely different way the two medusoid forms develop; 

 Scyphomedusae are formed by way of transverse division or 

 strobilation, and Hydromedusae by way of budding. As for 

 the velumx, I have succeeded in showing that this auxiliary 

 swimming organ developed genetically from the bud, i.e., 

 out of the emxbryonic protective stratum, with a simultaneous 

 change of its function (Hadzi, 1909 b). 



Various sense organs, that in the two medusoid forms ap- 

 pear as new formations, can be found developed as early as 

 in Turbellaria (statocysts, light receptors [eyes], and chemo- 

 receptors); as in the Turbellaria we can find in the medusae 

 variants that show considerable differences. The main changes 

 that can be observed, above all in the mechanoreceptors, are 

 due to the transition to the new, specialized type of movement, 

 i.e. to the rhythmic contractions of the umbrella muscles; 

 this, however, is not true for the movement of th e manubrium 

 w^hich had previously been found in the oral cone of polyps. 



In their progressive evolution the Scyphomedusae have 

 made greater advances than the Hydromedusae; this can be 

 partly attributed to their greater size. This, however, must not 

 be interpreted as if the Scyphomedusae had been developed 

 out of the Hydromedusae; in reality their developments have 

 been parallel and they had proceeded from two completely 

 separate initial stages. The Scyphomedusae are, in my opinion, 

 phylogenetically older, because the Scyphozoa themselves are 

 older and must be interpreted as animals that had evolved 

 from the Anthozoa. In the Scyphomedusae a locaUzed agglo- 

 meration of various sensory organs (rhopalia) can be observed 

 to have taken place in certain parts of the umbrella margin 

 that are separated from each other at regular distances. As 

 a consequence of this we can see an accumulation or 

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