PREVIOUS INTERPRETATIONS OF CNIDARIA 125 



only individual nerve cells but also whole coherent nerve 

 systems and sensory cells. These results have been arrived at 

 first by means of usual methods (serial sections, and the staining 

 of sections with the iron haematoxyhn) which have been used 

 by O. and R. Hertwig (1878), K. C. Schneider (1890), M. Miya- 

 shima (1898) and finally by means of the specific intra vitam 

 colouring with the methylene blue (Hadzi, 1908). This has 

 disproved Kleinenberg's theory of nerve-muscle-epithelium 

 cells which has been mentioned above, in spite of the attempts 

 made by Eimer to support it. They, and numerous other schol- 

 ars after them (among these I mention especially P. Groselj 

 [1909] who has suceeded in intra vita?n staining the elements and 

 mutual relationships of the nerve systems of anthopolyps) have 

 established the generally valid fact that there can be no doubt 

 that Cnidaria possess a nervous system, though in the form of a 

 nerve net which can be locally denser and which can even lead,, 

 though in medusae only, to the formation of nerve chains and 

 to the local accumulation of ganglion cells into small ganglia. 

 Yet, on the other hand no typical nervous system with one 

 centre, the brain ganglion, and with regular nerve trunks can 

 be found anywhere in Cnidaria. 



It has been easy to conclude— and such a conclusion has 

 really been made and it is still generally accepted— that in 

 Cnidaria w^e can find the primary primitive conditions of the 

 nervous system which has been further developed by way 

 of a progressive evolution into the typical nervous system on a 

 higher level of evolution. G. Wolf (1903), as has already been 

 mentioned, has even tried to construct the origin of the 

 nervous system in the following way: nerve cells have been 

 developed, according to this interpretation, from the epi- 

 thelially placed sensory cells by way of the latter sinking into 

 the subepithelium; in this way it has been believed that 

 the theory of synapses, or of a secondary conjugation of pri- 

 marily individual nerve cells, has been confirmed. 



It cannot be doubted that the nervous system found in the 

 polyps of Cnidaria (we will first discuss the primary form and 



