THE INFUSORIA 



181 



in advance of its contact with the entire body. As a result of this, 

 responses may be made before the cell becomes wholly surrounded 

 by new conditions, as when it approaches an area containing differ- 

 ent chemical compounds in solution. The possibility of such 

 " advance information " may be demonstrated by watching a 

 paramoecium as it approaches a drop of India ink (Fig. 96 C). 





Fig. 97. — Reactions of paramoecium to salts, acids, and heat. 



A, method of introducine a chemical into a slide of infusoria. B, slide of paramopcia 

 four minutes after the introduction of a drop of 5 per cent NaCI. The drop remains empty. 

 C, a slide of paramoecia is heated to 40 or 45 degrees C, then a drop of cold water 'repre- 

 sented by the outline a) is placed on the upper surface of the cover-glass. The animals 

 collect beneath this drop, as shown in the figure. D, coUectiou of paramoecia in a drop 

 of 1,30 per cent acetic acid. (From Jennings, "Behavior of the Lower Organisms," 

 reprinted by permission of Columbia University Press.) 



The power of receiving stimulation by "sampling" in advance is 

 no doubt useful to the animal in nature, and is presumably utilized 

 under conditions of experimentation when the cell responds (Fig. 

 97 B) l)y keeping out of a drop of | per cent salt solution, or, having 

 entered a drop of acid (Fig. 97 D), remains trapped therein because 

 it responds negatively whenever it again comes in contact with 

 the surrounding water. The avoiding reaction is, of course, the 

 means by which all such responses are effected. 



Responses to stimuli such as those described above are usually 

 known as tropisms; although, like the term " instinct," this word 

 has been used in so many ways that some investigators would 

 prefer to abandon it entirely. Thus, a response to light is positive 

 or negative phototropism; a response to mechanical contacts, 



