126 THE EVOLUTION OF THE METAZOA 



the basic generation of Cnidaria, i.e. the polypoids as a morpho- 

 logical and a physiological whole) really represent the simplest 

 organization that can be found in the entire animal world. The 

 only problem is whether this simplicity be primary, or a result 

 of a retrogressive development from an earlier higher level 

 of organization. In an analysis like ours we must take into 

 consideration the function of the nervous system as that of an 

 intermediary between the sensory cells or sensory organs as 

 receptors, and the muscles as effectors. I have already shown 

 with regard to the muscle system, that it had gone through 

 a generally retrogressive development, with the exception of 

 that specialization which is appropriate to the sessile way 

 of life. To my knowledge not a single complex sensory organ 

 has been found so far anywhere among the polyps of Cnidaria, 

 nor has there been published a description of such an organ. 

 There have always been only individual sensory cells ob served 

 in all the exposed parts of the skin stratum or even in the gut 

 epithelium. It is therefore not surprising if the absence of a 

 distinct nerve centre had been formerly attributed by the 

 brothers Hertwig (1878:185) to the absence of sensory organs. 

 They were right, with the exception that they should have 

 added that this absence of sensory organs has been a result of 

 the sessile way of life, similarly as has been the case with the 

 simplification of the muscular system. 



In my opinion there are two facts which have not been 

 satisfactorily taken into consideration in the interpretation of 

 the nervous system that can be observed in the polyps of 

 -Cnidaria. That idea occurred to me when I succeeded in selectiv- 

 ely intra vitatn staining the nervous system together with 

 the sensory cells that occur in green Hydra. This system appears 

 to be cytologically-histologically strongly differentiated, in 

 spite of its actually quite simple general structure. On the basis 

 of this fact alone one could conclude that the general simplicity 

 of this nervous system cannot be a primary phenomenon. 



The second fact that should be mentioned in this connec- 

 tion is that the "nerve net" of the polyps of Cnidaria is not 



