84 



THE GENESIS OF SPECIES 



[Chap. 



utterly remote from that on wliicli the animals of the 

 higher division provided with a spinal colunm are con- 

 structed. And indeed no transitional form (tending even 

 to bridge over the chasm between those two groups) has 

 ever yet been discovered, either living or in a fossilized 

 conditioi).^ 



A. B. 



CUTTLE-FISH. 



A. Ventral aspect. B. Dorsal a.spect. 



Nevertheless, in the two-gillcd Cephalopods (Dihran- 

 cliiafa) we find the brain supported and protected by a 

 cartilaginous cranium. In the base of this cranium are 

 two cartilaginous chambers. In each chamber is a mem- 

 branous sac containing an otolith, and the auditory nerves 

 pass from the cerebral ganglia into the cartilaginous cham- 

 bers to reach the auditorv sacs. ^Moreover, it has been 



^ Tliis remark is made without prejudice to possible affinities in the 

 direction of the Ascidians, — au affinity which, if real, would be irrelevant 

 to the question here discussed. 



