The Senses and the Nervous System 115 



like a sense of taste operating at a distance. It helps the ani- 

 mal to find what is good for it to take into its mouth. 



The catfish is an excellent example of this. The catfish 

 often feeds on detritus lying in the muddy bottom. It seems 

 to have given up using its eyes to locate food, but it wears 

 on its chin some projections called barbels, which are pro- 

 vided with taste-buds. They are auxiliary tongues. As soon 

 as food is put into the animal's tank, it shows excitement, 

 and begins to swim about over the bottom. This is due to 

 smell. But it is not until a barbel touches a fragment of food 

 that it is seized and eaten. This is taste. And some catfish 

 even go so far as to have taste-buds all over their bodies. 

 The remarkable creatures can actually taste with their tails 

 — doubtless a useful adjunct when searching for food in dark 

 waters. 



The sharks have a very keen sense of smell, on which they 

 depend entirely for locating food. Put a broken-up crab into 

 a dogfish's tank (a dogfish is a small species of shark), and 

 it starts swimming in a rough figure eight. Gradually it 

 draws near, but it is not until it is right on top of the bait 

 that it seems to see it and grasp it. Its eyes, as you recall, 

 have little if any ability to focus for distance, and its vision 

 is probably poor. If the animal's nostrils are put out of com- 

 mission by stuffing them with cotton soaked in vaseline, it 

 shows no consciousness of the food at all. If only one nostril 

 is so treated, it finds the crab, but instead of making figure 

 eights, it circles in one direction only, with the unobstructed 

 nostril on the inside of the circle. This complete dependence 

 on smell may be the explanation of why sharks aie apt to 

 leave potential prey alone unless it is wounded. Their eyes 

 are incapable of telling them that food is at hand, and they 

 must wait until the odor of flesh reaches their nostrils. 



Few fish have as keen a sense of smell as this. Some, like 

 the cichlids, with only one opening to each nasal sac, appear 



