2o6 THE SENSES OF ANIMALS 



by sound, sight, or the other senses. It is surprising that 

 substances diffuse through the water with such speed, for 

 we know that when a lump of sugar is dissolved in a cup 

 of tea the concentrated solution remains at the bottom until 

 it is stirred to mix it evenly. Nevertheless some molecules 

 must break away and travel throughout the whole, for a 

 suitable flavour introduced into the water several feet from a 

 fish is noticed almost at once. Newts and other amphibia 

 that live for long periods or permanently in water have a 

 similar taste-smell faculty. 



Do fishes that live in the sea taste the salt? It is unlikely 

 that they do as long as they are in sea-water with the normal 

 concentration of salt, but it does not follow that they cannot 

 taste salt. In the first place any nerve-ending that is stimu- 

 lated above a certain maximum suffers from fatigue and 

 stops sending impulses to the brain ; even our eyes and ears 

 refuse to function if over-stimulated, and thus we speak of 

 a blinding flash or a deafening noise. But there is another 

 reason for not noticing a constant stimulus, and this probably 

 fits the case of fish in sea- water better. A sense receptor may 

 be working perfectly and sending a stream of messages to 

 the brain, but if the brain is not interested it ignores them 

 and they remain unnoticed. When you are sitting reading 

 you do not notice the ticking of the clock on the mantelpiece, 

 although a message telling you of each tick is being sent to 

 your brain. If the clock stops you immediately notice it be- 

 cause the constant stream of messages that you have been 

 ignoring has ceased, and you become aware of the change. 

 Similarly fish in the sea probably do not notice the salt, but 

 should they move into water of greater salinity, or of less as 

 where a river runs into the sea, they probably taste the 

 difference at once. 



Most of the invertebrate animals that live in the water, 

 apart from crustaceans, insects and their relatives, have soft 

 skins — even sea-urchins have a thin covering of skin on the 

 outside — and their skins, like those of fishes, are covered 

 with chemo-receptor and other sense organs. The simpler 

 kinds of animals respond to the presence of certain chemi- 

 cals by moving towards or away from them, but their 

 structure is so primitive as to preclude the possibility of their 



