THE BREATHING APPARATUS, ETC., OF FISH 29 
lobe going to the left eye, and vice versa, but are not united ; 
but in Paleichthyes—as in the sturgeon—the two nerves cross 
and are fused together at the point of crossing, just as they are 
in the higher vertebrates. 
The question whether fish can hear is often debated among 
anglers as hotly as whether they can distinguish colours. As 
The sense every kind of fish, except the lowest of all, the 
of hearing. Jancelet (Brachiostoma lanceolatum),* is provided with 
a special acoustic apparatus, it seems unreasonable to deny 
that fish possess the faculty of hearing. Water surface is well 
known to facilitate the transmission of sound horizontally 
through air, but it remains in doubt in what degree atmo- 
spheric sound-waves communicate themselves to, and are 
transmissible within, the water.+ At all events, the acoustic 
apparatus of fishes is not on the same plan as that of terrestrial 
vertebrates. The ear has no tympanum or drum, neither 
is there any external orifice to the auditory chamber. In 
Teleostean fishes there is a labyrinth consisting of a vestibule 
connected with three semicircular canals, filled with fluid and 
contained among the cranial bones or within the cranial cavity. 
On each side of the base of the cranial cavity is placed a sac 
communicating with the vestibule. Sometimes each sac is 
divided into two unequal receptacles, each containing a stony 
concretion termed an ofo/ith, and in the vestibule is another of 
these concretions, in contact with the ends of the acoustic nerve. 
In some species communication between the ear and the outer 
world is facilitated by openings through the skull, closed 
externally only by skin or very thin bone. In the perches, 
* A creature with colourless blood and no brain, excluded on that 
account by Haeckel from the branch (Zyéa) of fishes, and isolated among 
vertebrates as the branch Acrania, or brainless animals. With less 
apparent reason Haeckel denied a place among Fishes to the Cyc/ostomata 
(lampreys), which he constituted a separate branch, Monorrhina. 
+ An instance in point from my own observation will be found on 
page 56. 
