GEOTAXIS 29 



Crustaceans are able to detect food at some distance and 

 direct themselves toward it, but one may question if such 

 phenomena should be classed under the head of true chemo- 

 taxis. Similarly with the olfactory reactions of ants. 

 These insects follow a scent track with considerable accuracy. 

 In this way they may be guided to food discovered by other 

 ants and find then- way back to the nest. The tendency to 

 follow these tracks is doubtless instinctive, and so also is the 

 action of a dog in following the trail of a rabbit. The be- 

 havior of the dog is on a much higher level than a mere 

 tropism, and it is probable that the behavior of the ant is 

 also, but to a less degree. But where to draw the line 

 between such actions and chemotaxis proper is perhaps 

 capable of only an arbitrary decision. 



GEOTAXIS 



Gravity exercises a directive effect upon the position and 

 movements of many animals while in other forms it has little 

 orienting power. The protozoans Euglena and Chlamy- 

 domonas commonly swim upward in the dark as well as in 

 the light, but this reaction is checked at a low temperature 

 of 5 or 6 C. Massart studied the geotactic movements of a 

 number of unicellular plants and animals by placing them in 

 a capillary tube open at either end so as to render the supply 

 of oxygen the same above and below. He found that the 

 sense of the geotactic response varies in allied species of 

 the same genus, as for instance, Spirillum some of which 

 are positive and some negative under the same conditions. 

 The sense of the response could be changed in some cases by 

 temperature. Thus Chromulina, which is negative at 15 

 to 20 C., becomes positive at a temperature of 5 to 7 C. 



The geotaxis of Paramoecium is quite variable and often 

 quite feeble. Generally there is a tendency to swim up- 



