350 AMPHIBIA xn. 25- 



open pit. In newts they are present in the larvae, which are aquatic, 

 but are covered by epidermal layers during the first post-larval stage 

 during which the newt lives on land. In the final aquatic adult stage 

 the organs reappear. 



The skin, of course, also contains tactile organs, and in addition is 

 often sensitive to chemical stimuli. This chemical sense is mediated 

 by fibres running in the spinal nerves, not by special elements such as 

 the taste-buds found spread out over the body in fishes. The skin is 

 also sensitive to heat and cold, and there is some evidence that these 

 senses are served by fibres different from those that mediate touch, 

 pain, or the chemical senses. Histologically, however, there is little 

 sign of the development of the special sensory corpuscles that are 

 so conspicuous in the skin of birds and mammals. All the nerve- 

 endings are of the type known as 'free nerve-endings', except for a few 

 touch corpuscles on special regions such as the feet. In this the 

 amphibia again resemble the fishes and show less differentiation than 

 do the higher animals. 



The taste-buds on the tongue and palate are probably able to 

 respond to the presence of only two of the four types of substance 

 that are discriminated by mammals. Applications to the tongue of the 

 frog and recordings of nerve-impulses in its nerves show that there 

 are chemoreceptors present able to respond to salt and sour substances, 

 but that no reaction is given to substances that in mammals are classed 

 as sweet or bitter. 



The olfactory organ functions both on land and in the water, special 

 mucous glands being present to keep it moist when in air. A continual 

 circulation of water or air is maintained over the olfactory epithelium 

 by cilia or the movements of respiration. The internal nostril may 

 have originally developed from the double nostril of fishes, in order to 

 make a circulation around the olfactory receptors possible. Jacobson's 

 organ is a special diverticulum of the olfactory chamber, serving to 

 test the 'smell' of food in the mouth. 



The Apoda, being blind, have a great development of the sense of 

 smell, including a hollow tentacle or olfactory tube. 



26. The eyes of Amphibia 



Provided that certain requirements are met the air gives more scope 

 for the use of photoreceptors than does the water. Light is trans- 

 ported with less disturbance through the air and image formation is 

 facilitated by the refraction of the air-corneal surface. The amphibia 

 have exploited these advantages and sight has become the dominant 



