354 A. A. W. HUBRECHT 



Nemertines (as was already advocated in my paper " zur Ana- 

 tomie und Physiologie des Nervensystems der Nemertinen/' 

 Amsterdam, 1880), then the double proposition above enunci- 

 ated necessarily leads to the conclusion that the spot just 

 mentioned corresponds to that part of the verte- 

 brate brain where the hypophysis (proboscis) bends 

 upwards towards the central nervous apparatus and 

 where the notochord (proboscidian sheath) termi- 

 nates, i.e. the region of the primitive fore-brain. 

 This proposition at the same time implies the homology be- 

 tween the vertebrate fore-brain and part of the nervous lobes 

 of Platyelminth ancestors. 



It remains to be further inquired into — and the facts as they 

 lie before us are very suggestive in this direction — whether 

 perhaps the distinction between the two pair of lobes as they 

 are present in most Nemertines may not have been perpetuated 

 in the vertebrates, these superior lobes (after dorsal coalescence 

 of the two halves of the nervous system) becoming the fore- 

 brain, the inferior ones the equivalents of mid- and hind-brain. 

 The following two points are in favour of such an interpre- 

 tation : (1) the nerves for the higher sense organs, eyes,^ and 

 olfactory (?) j)its start from the superior brain lobes; 

 (2) the strong nerve which on both sides supplies the an- 

 terior (respiratory, M'Intosh !) region of the oesophagus, 

 and for which in a former paper I have proposed the 

 name of N. vagus, takes its origin in the inferior lobes 

 (figs. 3 and 5). 



Upon the dorso-median coalescence of these inferior lobes 

 and of the lateral stems, above the intestine and the probos- 

 cidian sheath, the latter must have become severed anteriorly 

 from its connection with both nerve-system and proboscis. 

 Might not the fact of the anterior end of the notochord being 

 bent upwards in several of the lower Elasmobranchs (cf. 



' It is of course understood that tlic ectodermal eyes of Nemertines arc 

 not directly comparable to the myelouic vertebrate eye. However, it is im- 

 portant that GralF has already succeeded in demonstrating true cerebral eyes 

 in other Plutyclmiuths (' Mouogr. der Turbellarien ') ! 



