COMPARATIVE ZOOLOGY. 
178 
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\t“ w “L N r s “ cavity, are distributed over a moist 
cavity. 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 
wanting. Among Birds, Waders have the largest olfac¬ 
tory nerves. It is most acute in the carnivorous Quad¬ 
rupeds, and in some w T ild 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 auditory nerve. The 
vibrations of the fluid are usually strengthened by the 
presence of minute hard granules, call¬ 
ed otoliths. Most Invertebrates have 
no higher apparatus than this; and it 
is probable that they can distinguish 
one noise from another, but neither 
. Fig. 150—Ear of a Mol- 
pitch nor intensity. I he organ is gen- lusk (Oycias), greatly en- 
erally double, but not always located larged ’ 
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 
