CHAPTER 12 



Chemoreceotion 



INTRODUCTION 



ANY ORGANISMS Utilize sensitivity to the chemical consti- 

 tuents of the environment in the detection of food, 

 initiating the necessary orientation to food, and in regu- 

 lating the feeding habits and the ingestion of foods, and in the avoidance of 

 chemically unfavorable environments. Among insects and some other animal 

 groups chemoreception plays an important part in reproductive behavior, i.e., 

 attraction of male to female, and in regulating deposition of the fertilized eggs, 

 Chemoreception is, therefore, an important adjunct to the other sensory activi- 

 ties of organisms, all of which are essential for survival and propagation of 

 the species. 



The sense organs responsible for chemoreception have not been completely 

 identified, but all the evidence seems to indicate three types which differ in 

 sensitivity as well as in the role they play in animal orientation. The proto- 

 zoans and sponges exhibit no specific chemoreceptors and it is generally agreed 

 that, in these phyla, chemosensitivity is a consequence of the general property 

 of irritability inherent in living matter. This general chemical sensitivity 

 has been retained by the higher metazoans, including the vertebrates, al- 

 though among the latter Parker^^ has suggested that specific receptors, free 

 nerve endings in the skin or mucous membranes, are involved in general 

 chemical sensitivity.'- The outstanding characteristics of this general chemi- 

 cal sense are its low order of sensitivity and the negative avoiding reaction 

 which is typically elicited. 



In free living planaria, in insects, and in the vertebrates, there is excellent 

 evidence for the existente of specialized chemoreceptors whose sensitivity is 

 greater and whose role in organismic orientation differs from that of the gen- 

 eral chemical sense. Koehler"*" has published evidence indicating the exist- 

 ence of chemoreceptors located in the region of the auricular organs at the 

 sides of the head of Planaria luguhris which serve to detect food quite remote 

 from the organism, and of chemoreceptors located in the central anterior re- 

 gion of the head which function in feeding when the organism has reached 

 the food. Chemoreceptors similar in function to those of the planarian have 

 been described and in part identified for the insects and for vertebrates, and 

 it is generally agreed that these chemoreceptors fall into two sensitivity groups: 

 those receptors which orient the animal to food at a distance being the most 

 sensitive, and those which initiate and regulate feeding in the proximity of 

 food being less sensitive. Even the latter sense organs, however, are much 

 more acute than those mediating the general chemical sense. 



The classification of chemoreceptors into three groups, which is necessary 



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