178 
COMPARATIVE ZOOLOGY. 
in the gaseous state. Many Invertebrates have this sense: 
Snails, e. g., seem to be guided to their food by its scent, 
and Flies soon find a piece of meat. In the latter the 
organ is probably located on the antennae. In Verte¬ 
brates, it is placed at the entrance 
to the respiratory tube, in the upper 
region of the nose. There the olfac¬ 
tory nerves, which issue from the olfac¬ 
tory lobe of the brain, and pass through 
the ethmoid bone, or roof of the nasal 
F on\te _ w 0 a\fofX N r s e ai cavit J> are distributed over a moist 
cavit y- mucous membrane. The odorous sub¬ 
stance, in a gaseous or finely divided state, is dissolved in 
the mucus covering this membrane. In Fishes and Rep¬ 
tiles generally, this organ is feebly developed; Sharks, 
however, gather from a great distance around a carcass. 
In the Porpoises and Whales it is nearly or entirely want¬ 
ing. Among Birds, Waders have the largest olfactory 
nerves. It is most acute in the carnivorous Quadrupeds, 
and in some wild herbivores, as the Deer. In Man it is 
less delicate, but has a wider range than in any brute. 
Hearing is the perception of sound. The simplest 
form of the organ is a sac filled with fluid, in which float 
the soft and delicate ends of the audi¬ 
tory nerve. The vibrations of the fluid 
are usually strengthened by the pres¬ 
ence of minute hard granules, called oto¬ 
liths. Most Invertebrates have no more 
complicated apparatus than this; and it 
is probable that they can distinguish one 
noise from another, but neither pitch 
nor intensity. The organ is generally 
double, but not always located in the 
head. In the Clam, it is found at the base of the foot; 
some Grasshoppers have it in the fore-legs; and in 
Fig. 150.— Ear of a Mol- 
lusk ( Cyclas ), greatly en¬ 
larged, showing the oto¬ 
lith in the centre of a 
cavity which is filled 
with fluid, and whose 
walls are lined by ciliat¬ 
ed cells. 
