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 
i in their search for food. This being so, the noc- 
wcney Cals froma, turnal habits of many species is readily intelligible, 
eatin eile and the sense of smell must also be very useful 
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 
