128 THE EVOLUTION OF THE METAZOA 



medusae, a secondary thickening or condensation of nerve 

 elements (nerve cells and fibres, neurites) into a nerve ring 

 which cannot therefore be homologous to the nerve ring 

 together with the upper gullet and lower gullet ganglia of 

 Polymera, because they do not possess real ganglia. The am- 

 bulacral nerve trunks spread radially out of the nerve ring. 



When zoologists abandon the accepted idea of a supposedly 

 primary nerve net in Hydra they will have to search for the so- 

 lution of the problem of the origin of the nervous system 

 somewhere else. In my opinion we must consider the Tur- 

 bellaria. This suggestion stands in opposition to what has been 

 taught by many comparative morphologists (cf. Hanstrom, 

 1928). The simplest conditions of the nervous system can be 

 found in Acoela and in the small Turbellaria that are related 

 to the former. There is no real foundation for the supposition 

 which has been frequently proposed that the Turbellaria are 

 secondarily simpUfied animals, an interpretation which seems 

 to be supported by the fact that their size has decreased. The 

 Turbellaria are freely moving, and they search actively both 

 for food as well as (even though hermaphroditic animals) for 

 their partners. Their primitive nervous system is situated close 

 under the skin and it forms a primitive nerve net without any 

 real nerve centres, without ganglia or nerve trunks. Without 

 being afraid that the future will prove me wrong (so far no 

 factual data about this have been available) we can expect that 

 this nerve net does not develop during the ontogeny by way 

 of an invagination of the skin epithelium cells (ectocytium), 

 but rather "on the spot," out of the "mesenchyme" cells. We 

 will return to this when we discuss the problem of the origin 

 of Eumetazoa. 



During the second stage of the development of Turbel- 

 laria a condensation of nerve cells and of nerve fibres into 

 longitudinal bundles that are mutually connected by way of 

 transverse commissures (the orthogon, according to Reisin- 

 ger) can be observed. At the same time a second plexus is 

 being developed more deeply in the interior of the animal 



