i78 



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 

 tlie etiimoid bone, or roof of the nasal 

 ^^ume-wrjfTJr.! 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 Eep- 

 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 acnte in the carnivorous Quadrupeds, 

 and in some wild lierbivores, as tlie 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 flnid 

 are usually strengthened by tiie pres- 

 enceof minute hard granules, called oto- 

 liths. Most Invertebrates liave no more 

 complicated apparatus tlian this ; and it 

 is probable that they can distinguish one 

 noise from another, bnt 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 Grasslioppers have it in the fore -legs; and in 



Fio. ISO.— Ear of a Mol- 

 liisk {Cyclas), greatly en- 

 larged, showing the oto. 

 lith iu the centre nf a 

 cavity which ia filled 

 with fluid, and whose 

 walls are lined by ciliat* 

 ed cells. 



