32 NERVOUS SYSTEM AND SENSE-ORGANS 



smell is keen the nasal cavities are large and complex, and folds 

 project into them which increase the surface over which olfactory 

 cells are distributed. These cells are frequently of the shape 

 represented in fig. 1040, from which it will be seen that from the 

 outer end a number of slender processes project into the nasal 

 cavity. In some Fishes, such as the ordinary bony forms, the 

 originally single nostril of each half of the nose 

 is divided into two apertures, which respectively 

 serve for the entry and exit of water, that appears to 

 flow continuously through the nasal cavity. There 

 can be no doubt that many fishes possess a very 

 keen sense of smell, and the experiments of Bate- 

 son have proved that some of them (e.g. Dog-Fish, 

 Conger- Eel, and Sole) are mainly guided by this 

 in their search for food. This being so, the noc- 

 turna -l habits of many species is readily intelligible, 

 Amphibian (Proteus), an d the sense of smell must also be very useful 



greatly enlarged . 1111 i i. t . 



in water of such depth that the light is dim. 

 In Vertebrates which live on land the courses taken by the 

 food which is swallowed and the air that is breathed are more 

 or less distinct. Each nasal cavity, in fact, opens at the back 

 into the digestive tube, and the natural way of breathing is 

 " through the nose ". This is clearly to the advantage of the 

 sense of smell, for the air which passes over the olfactory cells 

 is constantly being renewed, and the incoming current is con- 

 tinually bringing with it gaseous matter capable of being smelt. 

 An inward flow is greatly promoted by the act of "sniffing", as 

 we know from our own experience. 



BALANCE AND HEARING 



There are certain sensory structures among the Invertebrates 

 which though often classified as Auditory Organs have probably 

 nothing to do with hearing in the ordinary sense, but are con- 

 cerned with advantageous adjustment of the body as regards its 

 position in space. This is of the greatest importance in reference 

 to the maintenance of balance and the direction of movement. 

 They are stimulated by vibrations in the surrounding medium, 

 water or air as the case may be, and there can be little doubt 

 that they have furnished the material from which undoubted 



