630 A. A. W. HUBREOHT. 



points ia regard to which the coraparison can be further ex- 

 tended. And it must be recognised that^ if we should also 

 succeed in rendering more or less probable a coraparison in 

 secondary details, this might again be favorably interpreted 

 for the primary and more important part of the hypothesis. 



The search after these secondary points of agreement was 

 instituted by me when the question above alluded to presented 

 itself, viz. if any remnant could be traced of the central ner- 

 vous system of Nemertea-like ancestors, i.e. of the brain-lobes 

 and lateral stems, in those Vertebrate descendants in which 

 the medio-dorsal tract had become so preponderant as to give 

 rise to the unpaired medulla and brain. 



It is clear that if it shall be possible to trace any such rem- 

 nants, and to render their homology with the Nemertean central 

 nervous system probable, they will have to be sought for — 

 (a) in the head, as lateral more or less independent nerve- 

 centra, innervating sense-organs of the integument, and passing 

 posteriorly into parallel longitudinal stems ; or {b) in the trunk, 

 as longitudinal nerve-stems, in which the central character 

 should be somewhat less marked than in the anterior swelling, 

 but in which the original significance as parts of the central 

 system should still be indicated either by histological or by 

 embryological features. 



To these latter conditions nothing can answer in the Verte- 

 brate nervous system excepting the so-called ramus lateralis 

 vagi. It is present in all Vertebrates above Amphioxus, 

 long and important in the aquatic Ichthyojjsida, gradually 

 disappearing when the aquatic medium is exchanged for an 

 air-breathing existence, and finally only retained in the higher 

 Vertebrates as the inconspicuous ramus auricularis vagi. 



Its course is indeed strictly lateral, and has always been a 

 puzzle to anatomists. Stannius^ characterises the existence 

 and the course of this sensory nerve along the trunk down to 

 the tail as " one of the most interesting facts of anatomy." 



None the less startling is its development. Whilst Balfour 

 attempted in this respect to bring it on one line with the other 

 ' ' Das peripherische Nervensystem der Fische,' p. 108. 



