IOWA ACADEMY OP SCIENCES. 
31 
is seen to consist of a utricnlo-sacculns, imperfectly divided 
into two parts, into which open two canals, each with an 
ampulla containing a sense organ. Unlike the condition 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 Petrorayzon 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. Embry olog^/^* 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 Petrcmyzon 
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 they 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 iu his interpreta- 
tion of their presence. Thus we see that recent 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. 
im. W. Norris. Studies on the ear of Amhlystoma. Part I. Journal of Morphology, 
1892. 
