232 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY 



of these has yet to be worked out), and undoubtedly the 

 auditory organ. As the existence of both these series 

 constitutes part of the definition of a vertebrate, it follows 

 that they were both represented in that lengthy period 

 during which the vertebrate was becoming evolved out of 

 the invertebrate. And, indeed, it is probable that the 

 sense organs of the epibranchial line, which seem to 

 degenerate before the adult condition is reached, represent 

 some such invertebrate lateral sense organs as are found 

 in the segmental worms of to-day. We must, however, 

 avoid the error of comparing two latter-day structures 

 unless the strongest possible evidence be forthcoming to 

 show that they are genetically related, and we have, let 

 us hope, seen the last of Eisig's comparison of the 

 segmental sense organs of Annelids with the sense organs 

 found in the " mucous " canals of recent fishes. We may, 

 however, classify the cranial nerves according to whether 

 they supply gill clefts or lateral sense organs. Such a 

 classification would have the following form : — 



Branchial Nerves or Nerves 

 Supplying Gill Clefis. 

 Cleft ? 

 Trigeminus. Mouth, cleft. 



Facialis. Hyoidean, spiracular, or 

 ear cleft. 



Glossopliaryngcus. First branchial 

 cleft. 



Vagus. Remainder of branchial 

 clefts, of variable number. 



Lateral Nerves or Nerves 

 Supplying Epibranchial and 

 Lateral Sense Organs. 

 Olfactorius. Smelling organ. 

 Opticus. Paired (Motor oculi.\ 

 eyes. Involves 

 also the nerves 

 secondarily de- 

 veloped as thW Patheticus. 

 differentiation 

 of these sense 

 organs pro- 

 ceeded. ^ Abduccns. 

 Auditorius. Hearing organ. This 

 nerve may include also the series 

 of nerves that supply the sense 

 organs in the "mucous" canals 

 of fishes. 



Eyi 



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