A 78 COMPARATIVE ZOOLOGY. 



in the gaseous state. Many Invertebrates have this sense : 

 Snails, e. </., 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 

 cavity, are distributed over a moist 

 ca vity. 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 



,. , . . . FIG. 150. Ear of a Mol- 



comphcated apparatus than this; and it i u sk (Q/ctoo greatly en- 

 is probable that they can distinguish one ^ ^^^ 

 noise from another, but neither pitch cavity which is filled 



r with fluid, aud whose 



nor intensity. The organ is generally walls are lined by ciiiat- 

 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 



