IOWA ACADEMY OP SCIENCES. 31 



is seen to consist of ?^ utriculo-sacculus, imperfectly divided 

 into two parts, into which open two canals, each with an 

 ampulla containing? a sense organ. Unlike the condilion in the 

 Lampreys, or higher Vertebrates, the two canals unite with 

 each other without an unpaired connection, or commissure, 

 with the vestibule. Hence the failure heretofore to recognize 

 more than one canal. The ear of Petromyzon differs from that 

 of Myxine chiefly in the fact that the two canals are connected 

 with the membranous labyrinth at their point of union by an 

 unpaired- commissure. The two semicircular canals of the 

 Cyclostome ear correspond to the anterior and posterior canals 

 of higher Vertebrates. 



The anterior is connected with the utriculus, and the pos- 

 terior with the sacculus, at their ampullar ends. In other 

 vertebrates the connection of the posterior canal with the sac- 

 culus is lost at an early stage of development, so that the three 

 canals in the adult are connected only with the utriculus. 

 This, however, is not the ancestral nor the early embryonic 

 condition. Embryology" indicates that the vertebrate ear 

 early consists of two parts, an anterior utricular and a posterior 

 saccular region. This is the adult condition in the cyclostomes. 

 Ayers calls particular attention to the fact, which Ketel, 

 Hasse, and Retzius had already noticed, that in Petromyzon 

 there are two distinct endolymphatic ducts, a further striking 

 indication that the vertebrate ear is a two fold structure in 

 origin. Ayers, however, gave the first explanation of their 

 presence. That the existence of these two ducts is a funda- 

 mental characteristic, is indicated by the fact that thsy are 

 distinct from a very early stage of development. 



Unfortunately the material at my disposal does not give a 

 complete series of the development of the ear, but the stages 

 studied by me indicate that Ayers is correct in his iaterpreta- 

 tion of their presence. Thus we see that receat investigation 

 confirms the opinion of Ketel that the auditory organ of the 

 Cyclostomata is not an aberrant structure. Ayers may be said 

 to be the first and only one who has given a coherent explana- 

 tion of the structure and origin of the Vertebrate ear. 



i*H. W. Norris. Studies on the ear of Amblystoma. Part I. Journal of Morphology, 

 1892. 



