CONTACT CEPTORS 71 



veloped eye degenerates and disappears. A similar 

 transformation occurs in the free-swimming Ascidian, 

 which likewise begins life with a comparatively highly 

 differentiated nervous system, consisting of a brain, 

 a spinal cord and a well-developed eye. Like the 

 Brachiopod, it becomes attached to a stationary object 

 and the highly developed nervous system, of use in 

 its motor life, disappears. 



The chemical ceptors for the apprehension of stimuli 

 and the initiation of purely chemical reactions are 

 distributed throughout the inner tissues. Chemical 

 ceptors are found in the linings of the stomach and in- 

 testine, in the brain and in the medulla. They govern 

 respiration and circulation ; they govern hunger and 

 thirst ; and, by maintaining the standard of chemical 

 purity of the body, they govern energy transformation. 

 Upon adequate stimulation the chemical ceptors give 

 rise to reactions which are in all respects as specific and 

 adaptive as those initiated by the contact and distance 

 ceptors. Although they are among the most obscure 

 and elusive of adaptive mechanisms, the chemical 

 ceptors present some of the most striking examples of 

 specific reflex action in the organism. 



Contact Ceptor Mechanisms 



In the functions of contact, distance and chemical 

 ceptors is to be found a remarkable confirmation of the 

 law of specific response, a typical illustration of which is 

 the adaptive reaction of Venus' fly-trap. 



Wherever in the organism mechanisms for the recep- 

 tion of contact stimuli exist, they are in type and func- 

 tion specific to the biologic needs of the area in which 



