18 SMELL,jTASTE, ALLIED SENSES 



common use and the emphasis that it places on the organs 

 to which it is applied as receivers of environmental change 

 rather than as originators of impulses to sensation is 

 certainly a step in the right direction. 



Human receptors belong to one or other of two classes. 

 Either they are concerned purely and simply with the 

 excitation of reflex acts and take no part in the pro- 

 duction of sensations, in which case they may be called 

 activators, or they are at the same time effective in 

 arousing sensations, the elements of the intellectual life 

 and hence may be appropriately termed sense organs. 

 All receptors belong to either one or the other of these 

 classes though in some instances a certain degree of 

 temporary vacillation occurs. Hence it may be that these 

 classes exemplify in a way two receptive functions, one 

 of which predominates in one class and the other in the 

 other. How these functions are related can best be 

 gathered from the genetic history of receptors. 



3. The Genesis of Receptors. Eeceptors such as the eye 

 and the ear, the organs of smell and taste, and the more 

 diffuse sensory equipment of the skin, are found in all 

 the more complex animals. They abound in the verte- 

 brates, the mollusks,the arthropods, and to a less extent in 

 the worms. They may be said to occur even in the coe- 

 lenterates, as, for instance, among the jelly fishes, though 

 in the majority of these animals the receptors present a 

 diffuse condition more like that seen in the vertebrate 

 skin than in the vertebrate eye or ear. This diffuse state 

 seems to be characteristic of the receptors in the simpler 

 sessile invertebrates. The more complex animals such as 

 are capable of active locomotion exhibit almost invari- 

 ably specialized types of organs. 



