378 THE ORIGIN OF VERTEBRATES 
the pectens or flabella, then the origin of the auditory apparatus 
would present no difficulty. 
It is also easy to see that the formation of the parachordals from 
entochondrites homologous with the supra-pectinal entochondrites, 
would give a reason why the VIIth or opercular nerve is involved 
with the VIIIth in the formation of the auditory capsule, especially 
if the special sense-organ which gave origin to the auditory organ 
was originally a pre-opercular sense-organ such as the flabellum, 
which subsequently took up a post-opercular position like that of 
the pecten. 
THE EVIDENCE oF AMMOCGTES. 
As to the auditory apparatus itself, we see that the elaborate 
organ for hearing—the cochlea—has been evolved in the vertebrate 
phylum itself. In the lowest vertebrates the auditory apparatus 
tends more and more to resolve itself into a simple epithelial sac, the 
walls of which in places bear auditory hairs projecting into the sac, 
and in part form otoliths. Such a simple sac forms the early stage 
of the auditory vesicle in Ammoccetes, according to Shipley; subse- 
quently, by aseries of foldings and growings together, the chambers of 
the ear of the adult Petromyzon, as figured and described by Retzius, 
are formed. Further, we see that throughout the Vertebrata this sac 
was originally open to the exterior, the auditory vesicle being first 
an open pit, which forms a vesicle by the approximating of its sides, 
the last part to close being known as the recessus labyrinthicus ; in 
many cases, as in elasmobranchs, this part remains open, or com- 
municates with the exterior by means of the ductus endolymphaticus. 
Judging, therefore, from the embryological evidence, it would 
appear that the auditory organ originated as a special sense-organ, 
formed by modified epithelial cells of the surface, which epithelial 
surface becoming invaginated, came to line a closed auditory vesicle 
under the surface. This special sense-organ was innervated from 
a large ganglionic mass of nerve-cells, situated close against the 
peripheral sense-cells, the axis-cylinder processes of which formed 
the sensory roots of the nerve. 
Yet another peculiarity of striking significance is seen in connec- 
tion with the auditory organ of Ammocctes. The opening of the 
cartilaginous capsule towards the brain is a large one (Fig. 154), and 
