844 Comparative Animal Physiology 



several tries. A conditioning experiment which should be repeated was per- 

 formed by Plavilstchikov (reported by Razran"^-"^) on the colonial peritrich 

 Carchesitim with colored light as CS and a mechanical prod as UnCS. All 

 of 82 colonies were "conditioned" to respond to the light after an average 

 of 138 trials at 15 to 20 per day, and the habit was reported to be retained 

 and carried along when part of the colony was transplanted to another col- 

 ony! 



There is no evidence of conditioning among coelenterates, although the 

 tentacles of fed actinians fail to accept paper after several repetitions; this 

 may be sensory adaptation. Echinoderms, particularly starfish, show complex 

 behavior in feeding, righting, and escaping from narrow confines, but there 

 is no evidence for lasting modifiability except perhaps in changing dominant 

 arms after restraint.-^* No conditioning experiments have been reported on 

 coelenterates or echinoderms in which different sense modalities have been 

 used as CS and UnCS. 



In the flatworms and annelids there is clear evidence for dominance of 

 cephalic sense organs and nerve centers. There are numerous examples of 

 reversal of a response to one sensory stimulus by combining such a stimulus 

 with another which is normally more potent. A marine flatworm, for ex- 

 ample, which was quiet in darkness and active in light, was "trained" by 

 touchino each time it moved in the light to remain quiet on illumination.-^- 

 Specimens of Neanthes C^ereis') which normally responded negatively to 

 illumination or to touch "learned" to come out of their tubes in the light by 

 virtue of an UnCS of mussel juice presented at the time of the light or 

 touch. i°-- ^"-^ Reversals of normal sign of response to light were elicited by 

 tactile stimulation in Hydroids^'-^-' and in Lumhriculus.^^' The earthworm 

 is normally positive to low light intensities and negative to high; if this is 

 also true for other species the reversal in response (as in Neanthes^ may be a 

 rise in threshold for negative response. This hypothesis could be tested by 

 measuring conditioned responses at several light intensities. Quite a different 

 order of conditioning was achieved on one individual earthworm by Yerkes 

 ^^" and on several by Heck^" and by Swartz,-^^-"* using a T-shaped maze with 

 electric shock on one arm. The worms formed an association with the "cor- 

 rect" (shockless) direction of turning, the habit was not lost after removal 

 of the supraesophageal ganglion, and worms from which several anterior 

 segments had been removed learned the habit. Apparently the ventral gan- 

 glia can be conditioned. 



Sensory adaptation is familiar in molluscs, and in one conditioning experi- 

 ment on a snail, Physa, tactile stimulation of the foot, which normally in- 

 hibited chewing movements, presented in combination with UnCS of a let- 

 tuce leaf, elicited chewing movements.^'" Experiments with snails in mazes 

 have not been very successful. Among the cephalopod molluscs, individuals 

 of Sepia and Octopus learned not to eject their tentacles at a prawn behind 

 a glass plate on which a white circle was painted, while they continued to 

 shoot at prawns not accompanied by glass and circle.^'^^' "^"^ A normal sepia 

 goes around a corner after a prawn which has disappeared behind a barrier; 

 when the dorsal integrative portion of the brain mass, the verticalis com- 

 plex, is removed (p. 821), the sepia no longer hunts around a corner for 

 a prawn, although it can still learn not to eject its tentacles at a prawn be- 



