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 antennse. 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 

 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 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 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. The organ is gen- lusk (q/cfas), greatly eu- 

 erally 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 



