30 IOWA ACADEMY OF SCIENCES. 



canals, or at least any more than as ludiments. It was very 

 early recognized that two distinct forms of ear were to be found 

 in the group of Cyciostomata, the one found in the Myxine and 

 the other in the Lampreys. Mtiiler^ first gave any adequate 

 description of the ear of Myxine. Previously Anders Rolzius^" 

 had given a very meager description. Ketel attempted to show 

 that the ear of Myxine is genetically related to that of higher 

 vertebrates through the ear of Petromyzon as a connecting 

 link. Unfortunately he failed to recognize the existence of 

 semicircular canals in the ear of Myxine, consideriog the mem- 

 branous vestibule as merely a ring. Ibsen" had in 1846 recog- 

 nized a semicircular canal in Myxine and two ampullae. 



Ketel considered the Cyclostome ear as in an arrested stage 

 of evolution, and that it really represented an ancestral condi- 

 tion of the Vertebrate ear. He sought for traces of the third 

 or horizontal canal in Petromyzon, and believed he found it in 

 a sense organ connected with the crista acustica of the anterior 

 canal. The cochlea he found represented in the "sackartiger 

 Anhang" of the membranous labyrinth. Ketel failed to com- 

 pletely homologize the Cyclostome ear with that of the Verte- 

 brate type, because he did not recognize the existence of 

 semicircular canals in Myxine, and further, because, working 

 from the higher types downward, he had not grasped the idea 

 of the fundamental form of the auditory organ. Gustav 

 Retzius^' in 1881 recognized the existence of a single semi- 

 circular canal in Myxine; but he did not agree with Ketel as to 

 the relationships of the ear of the Cyciostomata. It remained 

 for Ayers^^ in 1892 to establish beyond question the rank of the 

 Cyclostome ear. Starting with the idea that the Vertebrate 

 auditory organ is composed of modified sense organs of the 

 lateral liue system, he shows almost beyond question that the 

 Cyclostome ear is not a degenerate structure, but rather repre- 

 sents an ancestral type. According to this interpretation, we 

 recognize in the Vertebrate ear iwo originally distinct parts, 

 an anterior utriculus and a posterior sacculus, with which, and 

 forming a part of, are a number of canals. The ear of Myxine 



9Loc. cit. 



10 Ytterligare Biclra.g tiU anatomien af Myxine glutinosa. Kongl. Vct.-Akad. 

 Handl Stockholm, 1824. 



11 Anatomiske Undersogelser over Orets Labyrinth, afsluttet af Forgattern i 1846. 

 i2Dus Geliororgan der Wirbelthiere I, Stockholm, 1881. 



Invertebrate Cephalogenesis, II. A Contribution to the Morphology of the Verte- 

 brate Ear, witli a Reconsideration of its Functions. Journal of Morphology, Vol. VI, 

 Nos. 1 and 2. 1893. 



