562 John Beard 



Nervous System. 



The nervous system apparently consists in the later developmeut 

 of a veutrally placed mass of cells , which diiring ali the stages of de- 

 velopmeut of legs is closely connected with the skiu. In spite of much 

 search I have been unable to find traces in the later stages of the func- 

 tional larvai nervous system , and believe it entirely disappears. In- 

 deed my researches on both larva and adult lead me to the conclusìon 

 that no supra-oesophageal ganglion is present in the adult, nor have I 

 been fortunate enough to find any trace of the oesophageal ring de- 

 scribed by Geaff^. So far as I can judge the nervous system of Mt/- 

 zostoma is as follo ws. In the larva it consists of a median thickening 

 of the epiblast of the apex of the praeoral lobe. In the later stages of 

 larvai life this larvai nervous system completely disap- 

 pears. In the adult the nervous system consists solely of a ventral 

 mass of cells and connective tissue, which mass as will be shown later 

 presents traces of an origin from the fusion of several ganglia. 



Bergh^ has shown that the »Scheitelplatte von Nephelis ist dem- 

 gemäß ein vergängliches Embryonalorgan, dessen Zellen sich über- 

 haupt nicht als Ganglienzellen ausbilden, und das Gehirn entsteht (wie 

 schon Semper angegeben) aus den von Anfang an paarig sich ent- 

 wickelnden und erst später über dem Schlünde sich vereinigenden 

 Kopfkeimen.« 



My own researches on Myzostoma lead me to accept the above 

 view. Though the »Scheitelplatte« of Myzostoma is functional as brain 

 in the larva , I believe it has nothing to do with the formation of the 

 adult nervous system. The larvai nervous system like the «Scheitel- 

 platte« of Nephelis is only a transitory embryonic organ. In the adult 

 no supra-oesophageal ganglion is developed and no rudiments of it are 

 laid down in the larva. In fact one is led to the conclusion that the 

 supra-oesophageal ganglion of Myzostoma in consequence of retrograde 

 development and degeneration has completely vanished from the indi- 

 viduai ontogeny. 



On the development of 31. cirriferum. 



In the preceding pages the development of M. glahrum has been 

 chiefly considered , and now this must be compared briefly with that of 



1 1. c. p. 56. 



2 R. S. Bergh, Thatsachen aus der Entwicklungsgeschichte der Blutegel. 

 Zool. Anz. 1884. Nr. 164. p. 93. 



