No. I.] THE VERTEBRATE EAR. 219 



auditory vesicle the organs are inclosed in a protecting vesicle, 

 and a development of further protective structures would be, to 

 say the least, a superfluity. If, then, the only rational view to 

 take of the whole matter of the further differentiation and great 

 complication of the auditory sense organ after inclosure neces- 

 sitates the conclusion that all vertebrate forms possessing an 

 internal ear, provided with canals and canal organs, must have 

 descended from ancestors possessing a closed canal system on 

 the surface of the body, I think that there is no escape from 

 this conclusion, that the three distinct types of lateral line 

 organs found in the Cyclostomes, the Elasmobranchs, and the 

 Holocephala, Ganoids, Teleosts, and Amphibia are modified 

 from the simple type of canal organs as they are still found in 

 bony Ganoids, most Teleosts, and many of the Urodele Am- 

 phibia. As a somewhat parallel case where intricate structures 

 are developed instead of the simple ones as had been supposed 

 on a prio7'i grounds to have preceded and led up to the forma- 

 tion of the complicated conditions, we have the phenomena of 

 karyokinesis, or indirect cell division. It is now known that 

 karyokinetic complications occur in the division of the uni- 

 cellular forms of the organic world, and it would seem that 

 direct cell division, whenever it occurs, is a shortening of the 

 ancient and, so far as we can now see, original method of cell 

 division. This view, as regards the homology of the canal 

 organs in the several vertebrate groups, is the one which most 

 recent writers who have dealt with the surface organs of the 

 Ichthyopsida have advocated on the basis of very different facts 

 from those which I have given here. 



It follows from what has been said above that the system of 

 canal sense organs is a very ancient one, since it must have 

 antedated the origin of the internal ear of the Cyclostomes. 

 For although absent from Amphioxus, it is proven to have 

 existed in the Cyclostomes by the presence of the canal organs 

 of the ear. The Cyclostomes do not now possess the canals 

 during adult life, though there is good reason to believe that 

 they are laid down in the young larvae. 



The Elasmobranch group shows many important and exten- 

 sive modifications of this system in both the development and 

 adult relations of these organs, and Balfour has pointed out the 

 probable cause of the changes. 



