136 JOHN BEARD. 
The following line of argument may, as in the case of the 
olfactory, be used for the auditory segment. The sense organs 
and ganglion connected with the ciliary segment are without 
doubt homologous with the sense organs and ganglion of a 
cleft-bearing segment such as the glossopharyngeal. The 
ciliary has no pre- or post-branchial nerves because no gill- 
musculature or cleft. The auditory segment has no pre- or 
post-branchial branch just as the ciliary, but its sense organs, 
ganglion, and nerve are just exactly like, and have the same 
structure as the sense organ, ganglion, and nerve of the ciliary 
segment. Therefore the auditory nerve, organ, and ganglion 
are homologous with the nerve, sense organ, and ganglion of 
the ciliary segment, and therefore are also the homologies of 
the nerve, sense organ, and ganglion of the glossopharyngeal 
segment. But the sense organ and ganglion of the latter are a 
branchial sense organ and its ganglion, therefore the auditory 
organ is also a branchial sense organ, and the auditory nerve 
the remnant of a segmental nerve. 
Tmmediately behind and somewhat overlapping the sensory 
thickening which gives rise to the facial branchial sense organ 
is a long and broad auditory thickening (fig. 23). Behind the 
outgrowth of the neural crest which forms the facial nerve there 
is at a certain stage a small short outgrowth, this is the rudi- 
ment of the auditory nerve (fig. 23). It soon reaches the audi- 
tory thickening, fuses with it (figs. 24 and 25), and the ganglion 
begins to be formed at the point of fusion, and probably from 
the thickening itself as a proliferation just as in other cases. 
Before the auditory involution has proceeded very far there 
is a considerable ganglion formed, and fused with the auditory 
thickening (fig. 29). At this stage the whole nerve, sense 
organ, and ganglion correspond exactly with the nerve, sense 
organ, and ganglion of the ciliary segment (fig. 8). 
Soon the involution is carried to such an extent that the 
auditory organ forms a sac, but it still opens on to the surface, 
and in Elasmobranchs remains so throughout life. Even after 
the formation of the sac cells continue to be given off from 
the thickening to form the ganglion (fig. 31). The later formed 
