104 SMELL, TASTE, ALLIED SENSES 



strated that the skin of the catfish Amiurus was sensitive 

 to sour, saline, and alkaline solutions, a condition that 

 was subsequently found to be true for the young of the 

 lamprey eel Ammocoetes Parker (1908b, 1912). In 1909 

 Sheldon published an account of the chemical stimulation 

 of the skin of the dogfish Mustelus, the most extensive 

 study of this kind thus far made. Sheldon found that 

 the whole outer surface of this fish was very sensitive 

 to acids and alkalis, less so to salts and bitter substances 

 and not at all to sugar solutions, a condition that in gen- 

 eral confirmed the results of earlier workers. Crozier 

 (1915) studied the mutual relations of salts of sodium, 

 potassium, and calcium as applied to the frog's skin and 

 was able to demonstrate ionic antagonism which led him 

 to conclude that in normal stimulation the surface of the 

 receptor must be penetrated by the stimulant. 



These observations warrant the general conclusion 

 that the outer surfaces of most fishes and amphibians are 

 open to stimulation by chemical substances of a mildly 

 irritating kind. It is probable that this capacity has 

 been retained by the air-inhabiting vertebrates in only 

 a very circumscribed and local way, namely on those 

 exposed or partly exposed mucous surfaces that reproduce 

 in their delicacy and moistness the characteristics of the 

 general outer surface of aquatic forms. From this 

 standpoint the restriction of the chemical sensibility of 

 the air-inhabiting vertebrates is the result of the drying 

 of their skins in consequence of an ancestral migration 

 from an environment of water to one of air. 



3. Nerve Terminals. The form of nerve terminal 

 that is concerned with the reception of chemical irritants 

 in the skin of vertebrates is well indicated in the catfish 



