vii. i8 TASTE AND SMELL 221 



pharynx but may occur on the whiskers and all over the body. They 

 are innervated by branches of the seventh, ninth, and tenth cranial 

 nerves, which may reach far backwards (Fig. 137). In some species it 

 has been shown that the fish is able to turn and snap at a piece of food 

 placed near the tail. This power is lost if the branches from the cranial 

 nerves are cut. In mammals taste-buds serve to discriminate only four 

 qualities (salt, sour, bitter, and sweet), most of our so-called 'tasting' 

 being in reality the smelling of the food in the mouth. In fishes, also, 

 the four taste qualities are discriminated by the taste-bud system, and 

 it has been shown that the minnow (Phoxinus) continues to make such 

 discriminations after the forebrain has been removed. Other chemical 

 discriminations are made by the nose, however, and can only be 

 performed with an intact forebrain. Thus Phoxinus tastes and smells 

 the same classes of substances as man does. The taste-buds are 

 exceedingly sensitive, the threshold for sweet substances being 500 

 times and for salt 200 times lower than in man. On the other hand, 

 some substances that are very bitter for us produce little reaction in 

 Phoxinus. 



In many fishes the nose is one of the chief receptors (macrosmatic). 

 There are two nostrils on each side, allowing for the sampling of a 

 stream of water (Fig. 119). The nose does not communicate with the 

 mouth, except in a few fishes that live buried in the sand (Astroscopus). 

 The sense of smell is used to find food and for recognition of the 

 sex of members of the same species. Minnows can be trained to give 

 distinct reactions to extracts made from the skin of other species of 

 fish living in fresh water. In the presence of 'alarm substances' pro- 

 duced by damaged skin of a member of the same species, minnows 

 (and other fishes) show a 'fright reaction', scattering and refusing food. 

 The state of development of the nose is very varied. It is large in 

 macrosmatic solitary predators such as Anguilla and in many schooling 

 species that also have well-developed eyes {Phoxinus, Gobio). Daylight 

 predators, on the other hand, are microsmatic (Esox, Gasterosteus). 

 Other evidence shows that fishes can discriminate between the smells 

 of water plants and between the waters of different streams. It is 

 likely that this provides part of the mechanism by which salmon 

 return to the stream in which they were born, having been conditioned 

 as fry to the smell of its water. It has been suggested that they might 

 be decoyed to return to a stream other than that where they were 

 hatched by conditioning them as fry to a substance such as morpholene 

 to which they have a high sensitivity although it is neither an attract- 

 ant nor repellant. 



