lS2 THE BEHAVIOR OF LOWER ORGANISMS. 



straightening out or bending of pseudopodia as described on p. 179- 

 After becoming completely transferred to the surface of the solid the 

 form may differ much from that of the floating Amoeba. Fig. 65 illus- 

 trates such a reaction. A floating Amoeba will thus spread out on the 

 substratum, on the surface film, or, so far as possible, on small masses 

 of debris suspended in the water. 



An Amoeba which is moving along a surface also shows at times a 

 positive reaction to mechanical stimuli by turning toward small objects 

 with which it comes in contact at one side of the anterior end. This reac- 

 tion takes place very frequently in the normal locomotion of Amoeba, 

 but I have not been able to produce it experimentally by touching one 

 side of the animal with a glass rod. This is because it is difficult to 

 give a touch so light that it shall not induce the negative reaction. I 

 shall give a detailed account of reactions that probably belong here in 

 connection with the account of food reactions (pp. 196-202, and Figs. 



73-76). As will there be shown, the reaction is often long continued 

 and rather complicated. 



Le Dantec (1895) gave a good account of the positive reaction of 

 Amoeba, as shown in its spreading out on solids. 



NEGATIVE REACTION. 



I have studied the negative reaction to mechanical stimuli by touch- 

 ing a spot on one side or end of the animal with the tip of a fine glass 

 rod. A glass rod may easily be so drawn out that its tip is as fine as 

 the tip of a pseudopodium, and with some practice it is possible to give, 

 under the microscope, in the open drop, very precisely localized stim- 

 uli with this. 



We will first examine the reaction to a rather strong stimulus at the 

 anterior edge of an Amoeba that is creeping forward with outspread 

 anterior end and contracted posterior end in the usual way. The tip 

 of the glass rod is thrust sharply against the anterior edge, producing 



* FIG. 65. Positive reaction to a mechanical stimulus in Amoeba, in side view. 

 A floating Amoeba comes in contact by one of its pseudopodia with a solid (); 

 it thereupon passes to the solid, withdrawing the other pseudopodia (b and c). 

 See text. 



