HEARING. 
617 
to doubt that they communicate vibrations in the invertebrates. 
When the cephalopod mollusks, with their highly developed 
nervous system, are reached, we find a 
membranous and cartilaginous laby- 
rinth. 
Among vertebrates the different parts 
of the mammalian ear are found in all 
stages of development. The outer ear 
may be wholly wanting, as in the frog, 
or it may exist as a meatus only, as in 
birds. The tympanic cavity is wanting 
in snakes. Most fishes have a utricle 
and three semicircular canals, but some 
have only one; and the lowest of this 
group have an ear not greatly removed 
from the invertebrate type, as may be 
seen in the lamprey, which has a saccule 
with auditory hairs and otoliths, in com- 
munication with two semicircular ca- 
nals. Most of the amphibia are without 
a membrana tympani. The frog has (1) 
2 
thf 
Fie. 459. — Auditory vesicle of 
Geryonia (Carmarina), seen 
from the surface (after O. 
and R. Hertwig). Nand WN’, 
the auditory nerves ; Ot, oto- 
lith ; Hz, auditory cells; Hh, 
auditory cilia (type of the 
auditory organ of the Tra- 
chymeduse), 
a membrana tympani communicating with the inner ear by (2) 
a bony and cartilaginous lever (columella awris), and (3) an 
inner ear consisting of three semicircular canals, a saccule and 
Fie. 460.—Auditory vesicle of a heteropod mollusk (Pterotrachea) (after Claus). N,.auditory 
nerve ; Ot, otolith in fluid of vesicle ; Wz, ciliated cells on inner wall of vesicle ; Hz, audi- 
tory cells ; Cz, central cells. 
