ORIGIN AND STRUCTURE OF THE HEAD 57 



innervated each by a branch of a segmental nerve of the 

 ventral chain, that is by a spinal nerve branch. In Verte- 

 brates, on the contrary, the sense-organs of the trunk are 

 all innervated by the ramus lateralis of the nervus vagus, 

 a cranial nerve which, issuing from the brain, runs along 

 the whole trunk and tail, providing on its way every sense- 

 organ with a little side-branch. There is no question 

 about an innervation by spinal nerves in this case. Only 

 the partly transient lateral organs of the head have a 

 segmental innervation, to which we will refer below. 



If the lateral organs of Vertebrates are homologous to 

 those of Annelids, we must assume that in some way the 

 innervation by segmental nerves has been replaced by a 

 more concentrated innervation, having its centre in the head 

 We could imagine, for example, that a plexus-formation by the 

 original segmental nerves has begun from the head This 

 plexus extending progressively further backwards, has 

 resulted in the disappearance of the segmental communi- 

 cation of the lateral sense-organs with the corresponding 

 dorsal spinal nerves. This is also what ElSlG (1887, p. 53) 

 supposes, when he writes; "Die einfachste Voraussetzung 

 ware die. dass m dem Maasse als das Gehirn seine Function 

 als Centralorgan im werdenden Wirbelthiere auf Kosten der 

 relativen Selbstandigkeit der segmentalen Ganglienknoten des 

 Anneliden-Bauchstranges auszuuben fortfuhr (ElSIG of course 

 has in mind DOHRN's view, at that time prevalent, that the 

 brain is to be comoared with ihe cerebral ganglia, the spinal 

 chord I with the ventral ganglion chain. D.).sich zwiscren den 

 Asten der die Seitenorgane innervirenden Spinalnerven, zum 

 Behufe einer directeren Leitung der Erregungen, successive 

 Anastomosen ausbildeten, mit anderen VVorten, dass der 

 Seitennerv oder Ramus lateralis vagi nach dem Principe 

 eines Collectors zu Stande kam". Such a plexus- and 

 collector-formation is also seen to appear in other cases 

 where unity of function and a more direct communication 

 with the brain is required, e.g. in the innervation of the 

 dorsal fin and of the paired limbs of fishes. 



As stated by many authors, lastly by KLINKH.4RDT (1905, p. 

 475). the ramus lateralis grows out in a backward direction 

 within the epithelium and only afterwards detaches itself 

 from the latter. 



Beard versus EisiG.— Beard (1885). though formerly (1884) 

 adhering equally to the homology of the lateral organs in 



