120 BEHAVIOR OF THE LOWER ORGANISMS 



in the usual way, — showing the same ciliary movements as free indi- 

 viduals, — but might find itself stuck, and unable to escape. Doubtless 

 sometimes this condition of affairs is realized ; it is described, for ex- 

 ample, by Putter as present in the reaction of attached specimens of 

 Colpidium and some other infusoria. But in many cases this physical 

 factor will not account for the observed behavior. Infusoria in contact 

 may take different positions without difficulty, and could easily place 

 themselves in line with gravity, yet as a rule they do not do so. Attached 

 Stentors could easily bend into a position with anterior end away from 

 the light, yet their position shows no relation to the direction of the light 

 rays. There is nothing in the physical adherence to a surface that 

 should compel the animal to take a transverse position in the electric 

 current, rather than a position parallel to the current, yet this is what 

 occurs in attached specimens. It is clear that there is a physiological 

 factor involved. Contact with solids tends to make the animal act in 

 one way, the other stimulus in another; hence the two must interfere. 

 If we object, as some authors have done, to the admission that the contact 

 reaction interferes with the reaction to other stimuli, we are compelled 

 to admit in any case that the reactions to other stimuli do interfere with 

 the contact reaction, and one admission has as much theoretical signifi- 

 cance as the other. It is evident that when two agents influencing 

 the organism in opposite ways act simultaneously, the effect of one must 

 give way to that of the other, or the two must combine to produce a 

 resultant. It is impossible that each should produce its characteristic 

 effect. The interference of the contact reaction with the reactions to 

 other stimuli is one of the most striking phenomena to be observed in 

 the behavior of these lower organisms. It is always necessary to dis- 

 tinguish carefully the behavior of free swimming specimens from those 

 that are in contact with surfaces, for the two differ radically. 



3. REACTION TO CHEMICALS 



The reactions to chemical stimuli take place in all accurately known 

 cases through the typical avoiding reaction. As a rule the motor organs 

 of the infusoria, both flagellates and ciliates, act in such a way that a 

 current of water passes from in front of the animal to the anterior end 

 and mouth, as illustrated for Paramecium in Fig. 35. Thus when a 

 chemical is dissolved in the water, a "sample" of it is brought to the 

 most sensitive part of the body. If the chemical is of such a nature as 

 to act as a stimulus, the animal swims more slowly, stops, or moves 

 backward, turns toward the customary side (usually the aboral side), 

 until it no longer receives the chemical, then moves forward in the new 



