136 THE EVOLUTION OF THE METAZOA 



ancestors, and, on the other hand, when we declare these organs 

 of Cnidaria to be new formations. In details the structures of 

 these sense organs can show considerable differences from 

 each other, which does not surprise us because of the complex 

 nature of these organs and because they have been developed 

 under different circumstances. 



It is just in connection with the sense organs of medusae 

 that we can observe numerous and interesting examples of 

 the variability of these organs. Thus, the statocysts, as organs 

 of equilibration and stimulation, show considerable differences 

 in their structures not only if we compare the way they occur 

 in Hydromedusae on one hand, and in Scyphomedusae on the 

 other; there are also considerable differences in statocysts even 

 within Hydromedusae themselves. Sometimes, such organs are 

 developed with participation of the entoderm (as, for example, 

 in tentacles); at other times, however, they are of a purely 

 ectodermal origin; they can lie exposed, or they can be sur- 

 rounded by other tissue, they can be free or immersed, etc. 

 In Scyphomedusae, these marginal organs (rhopalia) are very 

 complex, i.e. they are composed of several different partial 

 organs. The same is true for the light receptors that can appear 

 in very different stages of development and which have clearly 

 been evolved several times and independently from each other. 



It should be mentioned, with regard to the retrogressive de- 

 velopment of the nervous system of polyps, that in hydro- 

 polyps this process has gone so far that we are hardly justified 

 to speak any longer of a nervous system because it appears as 

 a very sparse net only, and frequently we find even this last 

 remn?.nt of a former nervous system completely absent in 

 many parts of the animal body. It should be added here that 

 parallel to this sparsity of the nerve net, the "nervous tissue" 

 is moved from the intermediate layer into the skin layer, i.e. 

 to the base of the skin epithelium cells. 



A reversion of this process can be observed in the medusoid 

 forms— inasmuch as these themselves do not tend to retrogress 

 (Hydromedusae that have adopted the sessile way of life). 



