3] THIGMOTAXIS 105 



3. EFFECT OF MOLAR AGENTS IN DETERMINING THE 

 DIRECTION OF LOCOMOTION THIGMOTAXIS (STEREOTAXIS) 

 AND RHEOTAXIS * 



We have already seen that when a pseudopodium of an 

 amoeba is touched by a solid body it retracts. In this retrac- 

 tion the centre of mass is transferred to a new point. If the 

 stimulation is often repeated upon the same side, contraction 

 continues on that side, until eventually the amoeba will have 

 migrated a considerable distance. In this case the determina- 

 tion of the direction of locomotion is closely allied to the phe- 

 nomena of contraction as a result of stimulation, considered in 

 section 2. The retraction of the protoplasm which follows its 

 irritation is the cause of the migration of the amoeba in a defi- 

 nite direction. This direction is away from the touching body. 

 The response may consequently be called negative thigmotaxis. 



The phenomenon of negative thigmotaxis is widespread. 

 There are almost no free-moving organisms which do not move 

 away from contact or molar disturbance of an unusual or vio- 

 lent sort. Thus you may very definitely control the direction 

 of movement of a planariaii or a slug by touching the body 

 upon the side opposite the direction in which you wish it to 

 move. In such cases, also, there is first a contraction of the 

 body upon the irritated side. 



The opposite phenomenon of movement towards, or clinging 

 to, the irritating body positive thigmotaxis is less common 

 and therefore more striking. It has long been known, I imag- 

 ine, it certainly is an observation easily made, that an 

 amoeba which has come in contact with a solid body clings 

 close to it and moves over its surface. LE DANTEC ('95, 

 p. 211) has described the action in much detail. An amoeba 

 descending in the drop touches the glass slide first by a single 

 protruding pseudopodium. Next, the pseudopod elongates hori- 

 zontally, and at the same time affixation takes place, so that the 

 organism does not roll about when the water is agitated. The 



* Thigmotaxis, under the different form " thigmotropism " (from 

 "contact") was first applied to these phenomena by VERWORN ('89, p. 90); 

 stereotaxis, under the form " stereotropisin " (from o-repeos, "solid"), was intro- 

 duced by LOEB ('90, p. 28), and is practically synonymous with thigmotaxis. 



