.08 AVERS. [Vol. VI. 



3. It is proven that there is no essential difference between 

 the sense organs of the internal ear of the vertebrate group 

 and the superficial sense organs of the Ichthyopsida as they 

 exist in the surface canals of these forms. Both consist of 

 canals containing sense organs lying below the surface of the 

 body, and they may (as in ordinary canals and the ear in some 

 Elasmobranchs) or they may not (as in Savi's vesicles and the 

 ear in the majority of vertebrates) communicate with the sur- 

 face. Both forms may or may not be entirely inclosed in 

 cartilage or in bone. 



4. The semicircular catials of the ear are simply remnants of 

 the canal system of the surface, and although bent into shapes 

 more or less semicircular, they still retain their communication 

 with the (morphological) exterior by means of their terminal 

 {i.e. surface) pores or openings in the walls of the utriculo-sac- 

 culus, which in its turn often communicates with the surface 

 of the head by the endolymphatic duct or surface canal. 



5. The sense organs of the ampullae not infrequently divide 

 without causing the division of the canal, and even the crista 

 abortiva in some Elasmobranchs and Reptilia has divided, each 

 sense organ resulting being supplied with a distinct nerve 

 branch. The fact of the presence of two parts in his so-called 

 macula neglecta was not understood by Retzius, who first 

 observed it. 



6. When the division of the sense organ has proceeded so 

 far as to cause a separation of the previously unified nerve 

 trunk supplying it, we are justified in concluding that the num- 

 ber of sense organs in the group is at least equal to the number 

 of discrete nerve twigs supplied to the group. 



7. Each offspring resulting from the division of a canal organ 

 inherits a tendency to division and canal production. The 

 auditory canal sense organs of the second generation divide 

 symmetrically when considered in their mutual relations, but 

 individually the division is an asymmetrical one. The third 

 o-eneration shows the same characteristics. 



o 



8. Thus it is true that the development of the semicircular 

 canals in the ear is due to phylogenetic and mechanical causes, 

 and is not in the least a response to physiological necessities or 

 requirements, and conversely the canals have no active part in 

 the auditory function. They serve merely as chambers to hold 

 the hquid necessary to float the auditory sensory hairs. 



