2i6 Journal of Comparative Neurology and Psychology. 



appropriate centers. In cyclostomes neuromere vii is blank and 

 neuromere viii is related to the structures enumerated. In 

 gnathostomes neuromere viii is blank and neuromere ix is re- 

 lated to the same structures — not that neuromere viii in cyclo- 

 stomes or ix in gnathostomes lies in the same transverse plane 

 as the other structures of segment 7, but that it is con- 

 nected with those structures by the nerve roots. In Fig. 10 

 the relations are shown as in Petromyzon; in Table B the same 

 facts are presented by giving to each element its hypothetical 

 primitive position and stating the shifting which is supposed to 

 have taken place. 



In all craniata the neuromasts with their fiber components 

 and centers are added to the above structures. These now re- 

 quire some attention. 



The essential unity of the acustico-lateral system both 

 peripherally and centrally and the close relation of these organs 

 and centers to the general cutaneous system is assumed {dj, 68, 

 70). It is a natural inference that the neuromast system has 

 been derived from the general cutaneous. Further, it has been 

 shown by the work of numerous authors that the whole neuro- 

 mast system grows peripherally from t/ie vicinity of the auditory 

 pit as a starting point, that the fibers enter the brain at a com- 

 mon point near the auditory nerve, and that the same sensory 

 nuclei serve as the centers for all these fibers. It therefore 

 seems pretty clear that we may look for the origin of the neuro- 

 mast system as a modification of the general cutaneous system 

 of a given segment or small number of contiguous segments, 

 and that the center of the neuromast system was primitively in 

 about the region of neuromere vii, opposite which the auditory 

 pit now makes its appearance in the lowest known vertebrates. 



Although the neuromast nerves have usually been regarded 

 as dorsal rami (when they have not been given the dignity of com- 

 plete dorsal roots) a difficulty arises in the fact that the ganglion 

 cells of the neuromast rami are derived from ecodermal thicken- 

 ings, and that the placodes of nerves for the head lines are at 

 first in connection with such undoubted dorsal rami as the 

 ophthalmicus profundus and ophthalmicus superficialis trigemini. 



