200 THE SENSES OF ANIMALS 



modification of tlie nose unlike that of other animals. In 

 addition to the usual nose-passage with nerve-endings for 

 smell in its upper part, these creatures have a further arrange- 

 ment for smelling. Tv^o little pits lie in the roof of the mouth 

 towards the front; they are derived from the nose-cavity 

 during the course of development, and their linings are 

 richly supplied with nerve-endings similar to those in the 

 upper part of the nose-cavity. The pits, which are called 

 "Jacobson's organ", are used in conjunction with the forked 

 tongue. The tongue is not poisonous, nor is it a sting, as is 

 sometimes supposed. The well-known flickering action of 

 the tongue in snakes and lizards has long been regarded as 

 a means of testing the environment by the sense of touch. 

 Doubtless the tongue is a delicate tactile organ and often 

 functions by actually touching objects, but if a reptile is care- 

 fully watched the observer can easily see that frequently the 

 tongue merely flickers in the air without toucliing anything 

 and then is quickly withdrawn. The flickering tongue is 

 picking up odorous particles floating in the air, and trans- 

 ferring them to Jacobson's organ when it is withdrawn. In 

 many snakes there is a small gap in the lips at the front of 

 the mouth so that the tongue can be flickered in and out 

 without opening the mouth. Evidently Jacobson's organ 

 increases the sensitivity of the sense of smell, but exactly how 

 it does so is not known; it may be that the method of 

 breathing in these reptiles does not draw enough air into the 

 upper part of the nose-cavity to allow a quick perception 

 of air-borne particles. 



Although many of the frogs and toads find their food by 

 sight some of the amphibians use the sense of smell for this 

 purpose. The species that live wholly or part-time on land 

 have noses with nerve-endings in them of the type already 

 described. There appears to be some kind of sense of smell, 

 too, in the moist skin of some amphibia — the skin through 

 which much of the breathing takes place in some species, to 

 such an extent that certain salamanders have no lungs at all. 

 Those amphibians that live wholly in the water can scarcely 

 be said to have any sense of smell, if we mean thereby the 

 ability to detect air-borne vapours. The same remark applies 

 to the fishes, and we shall therefore defer considering the 



