448 Comparative Animal Physiology 



for clarity of discussion, poses the problem of names. In man chemorecep- 

 tors are classified as olfactory sense organs, gustatory or taste sense organs, 

 and the general chemical sense organs, in the order of their decreasing sensi- 

 tivity. The application of this terminology to the chemoreceptors of other 

 organisms not only has an anthropomorphic flavor but also leads to certain 

 genuine difficulties. For instance, in man olfactory sense organs are com- 

 monly stimulated by air-borne substances, and taste receptors are commonly 

 stimulated by substances in solution, whereas in aquatic organisms both types 

 of receptors are stimulated by substances in solution. One may well ask if 

 aquatic organisms can smell. Olfactory sense organs have also been referred 

 to as distance receptors, as opposed to taste receptors, which are stimulated 

 by contact. Yet the superficial taste buds of fishes may be stimulated by food 

 held near the flank of the animal. Taste receptors, by connotation, are asso- 

 ciated with feeding responses, but among the insects there exist chemorecep- 

 tors of equivalent sensitivity which function in oviposition. Dethier and 

 Chadwick-*' have partially resolved the difficulty by calling this group (taste 

 and ovipositor receptors) contact chemoreceptors. This name, however, im- 

 plies the essential condition of contact for this group of chemoreceptors only, 

 which is certainly not the case. The sensory end-organs mediating the gen- 

 eral chemical sense are also contact chemoreceptors, and, indeed, even the 

 olfactory sense organs are stimulated only by eventual contact of the stimu- 

 lating agent with the sense cell. 



In view of the fact that new names often serve to increase rather than to 

 dissipate confusion, the human terminology will be adhered to in the follow- 

 ing discussion, despite the above difficulties. The terms olfactory, gustatory, 

 and general chemical sense should, however, imply chemoreceptors of particu- 

 lar sensitivities rather than the particular sensations associated with these 

 names. '• 



CHEMORECEPTION AND BEHAVIOR 



Chemoreception in the Lower Invertebrates. Protozoa are known to react 

 positively to certain chemicals, negatively to others, and not to react to still 

 others. For example, various solutions applied by means of a capillary pipette 

 to a small portion of the surface of Am,oeha proteus produce pronounced 

 changes. Weak alkali causes the formation of a protuberance which develops 

 into a normal pseudopod and results in locomotion toward the alkali. Weak 

 acid or weak sodium chloride solution causes the formation of the protuber- 

 ance, but not of a pseudopod. Stronger acid or salt solution may cause pseu- 

 dopods to form on the side opposite the point of application. 



In Paramecium the cilia may be reversed by the presence of certain chemi- 

 cals in apparently the same manner that they are reversed by contact or by 

 rapid changes in temperature or in osmotic pressure, and reversal is usually 

 followed by turning toward the aboral surface and then by forward move- 

 ment in a different direction. It is the first stage of the avoiding reaction. 

 This results in aggregation in a slightly acid region if surrounding regions are 

 slightly alkaline. Similar effects probably occur in response to changes in 

 gas concentration and may account for many of the aggregations which are 

 often observed when Protozoa are studied on a slide under a cover slip. The 



