21 



— pierced so as to divide the brain into a supraoesophageal and a 

 suboesophageal portion which might be compared respectively to such 

 divisions in the Annehdan nervous system. At first sight this appears 

 like an admission that the Annelidan theory of the origin of Verte- 

 brates is untenable. But such is not the case. 



From a large number of researches on Annelida including those 

 of Bergh, Salensky and Kleinenberg, we know that the supra- 

 oesophageal ganglion of Annelids certainly arises independently of 

 the ventral chain, and that it only later becomes coupled on to 

 the latter, by the development of the oesophageal collar. 



Kleinenuerg's brillant researches also teach us how the per- 

 manent Annelidan nervous system arises through substitutions and partial 

 or entire disappearences of whole larval nervous apparatuses and sense 

 organs. And indeed after reading his beautiful work one is fully prepared 

 for one of the closing statements in it — that possibly the supraoeso- 

 phageal ganglion is entirely absent in Vertebrates. Personally I have 

 no hesitation at all in accepting this as probably true. But the 

 grounds for my belief or some of them I can only hint at here. They 

 arise out of as yet unpublished developmental researches. Briefly 

 stated I see in the development of the gill-clefts, with their special 

 sense organs and ganglia ^) all of which lie in the region which is 

 under the control of what is comparable to the ventral nerve cord 

 of Annelids, a probable cause of the disappearence of the supra- 

 oesophageal ganglion in the ancestors of Vertebrates — in a similar 

 way to that in which according to Kleinenberg, the dislodging and 

 destruction of the special larval ganglionic centres takes place in 

 the Annelida. 



In the ancestors of Vertebrates, by the development of the eyes, 

 and of the important gill sense organs, ganglia etc., the ventral chain 

 obtained control over a very extensive system of ganglia, sense organs 

 and muscles, and, having already, as we shall soon see, a control over 

 the mouth or Schlund, it deposed entirely the supraoesophageal ganglion 

 (and its sense organs). And, in fact, the entire raison d'etre of the 

 latter being thus disposed of, it naturally degenerated and, finally, 

 disappeared. If it be admitted that the supraoesophageal ganglion 



1) The cranial ganglia of Vertebrates are far more complicated jmorpho- 

 logically then has hitherto been recognized. In addition to parts which 

 appear to correspond morphologically to the posterior root ganglia of the 

 spinal nerves plus the sympathetic ganglia, they also contain the special 

 ganglia which are formed in connection with the gill sense organs. 



