ORIGIN AND STRUCTURE OF THE HEAD 49 



brain according to RacOVITZA have originated, but to each 

 of them corresponds a ganglion or a pair of ganglia applied 

 to a sense-organ. These are the "ganglions palpaires", which 

 belong to the "cerveau anterieur", the "ganglions optiques'" 

 and "antennaires", which belong to the "cerveau moyen'" and 

 the "ganglion nucal", belonging to the "cerveau posterieur". 

 Now the nuchal pits are certainly dorsal structures and 

 so are accordingly the olfactory lobes of the Annelid brain. 

 Of the eyes it is not so easy to say whether they belong 

 to the dorsal or to the ventral side of the prostomium. In 

 the adult Annelid they are usually situated on the dorsal 

 side, in the trochophora and in somewhat further advanced 

 pelagic stages we find them situated nearly laterally but 

 somewhat more to the ventral side. Often the ventral 

 I situation is even very evident, in Annelid as well as in 



I Mollusc trochophora-larvae. I therefore believe that the eyes 



must be regarded as originally ventral structures, and this is 

 in accordance with the circumstance, thatthey are closed round 

 when the ventral half of the apical plate folds in to become the 

 fore-brain in Vertebrates, while this is not the case with the 



(olfactory pits. Finally we may regard the "cerveau anterieur** 

 as teiminal and neither ventral nor dorsal in its origin. 

 Thus only the optic nervous centre of Annelids is involved 

 at the closing of the cerebral plate in Craniates and probably 

 may be found again in the optic ganglia of the retina (the 

 superficial granular and fibrillar layers) and in the region 

 of the chiasma. In front of the latter we find in lower 

 Craniates, especially in fishes, the strongly developed basal 

 ganglia as thickenings of the lamina terminalis — corres- 

 ponding to the corpus striatum of higher Vertebrates — while 

 the roof of the forebrain, the pallium, consists here merely 

 of a thin epithelium, without nerve cells. In higher Verte- 

 brates the pallium more and more develops, producing the 

 hemispheres which take over the function of the basal 

 ganglia as an olfactory centre, until in Mammals the corpus 

 striarum sinks into obscurity by the enormous development 

 of the pallium. Can we compare these basal ganglia with 

 one of the .three centres of the Annelid brain, especially 

 with the olfactory centre? I do not think so, at least not 

 directly, for the "Riechlappen" or "cerveau posterieur** 

 originate, as shown by KleinenbekG, in close connection 

 with the olfactory pits. Such is not the case with the basal 

 ganglia of Vertebrates ; they take their origin from the part 



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