Food Selections Among Lower Animals. 43 



More interesting than the fact that ameba can discriminate in 

 food is the observation that an auteba can sense objects at a dis- 

 tance. Not only is this true of sohible particles, but of insoluble 

 as well. A fragment of clean glass, one-twentieth millimeter in 

 diameter, is sensed by an ameba at a distance of one-tenth milli- 

 meter. That is^ the ameba moves toward the glass particle in a 

 direct line after changing its direction, if the particle is placed to 

 the side of the ameba's probable path. This has been observed 

 in a number of cases, so it cannot be said that it is a matter of co- 

 incidence only. 



Reaction to an object at a distance by an ameba is a remark- 

 able phenomenon, for this animal has no specialized sense organs 

 of any sort. This makes it difficult to determine how the presence 

 of the particle of glass is sensed before contact is made. Glass is, 

 of course, very slightly soluble, but it must be remembered that 

 the observation was made with the ameba moving on a glass sur- 

 face; so that even if the glass particle was soluble, its efifect on 

 the ameba was cancelled by the solubility of the glass surface 

 over which the ameba moved, the glass particle being a fragment 

 of the dish in which the experiment was made. See Fig. 2. Some 

 work has been done to determine in what way the ameba senses 

 insoluble particles at a distance, but no definite conclusions have 

 been arrived at. This work is still in progress. 



From the work on stentor and on ameba it may be concluded 

 then that the unicellular animals possess the power to discrimi- 

 nate in food, and that they exercise this power in a high degree. 

 The observations on ameba show also that an unlooked for sense, 

 that of becoming aware of objects at a distance, is present in 

 ameba. 



