Sensory Discrimination : Vision 123 



42. Vision in Coslenterates 



Turning to the coelenterates, we find that Hydra shows 

 no response to light other than a tendency to come to rest 

 in the more illuminated parts of the vessel containing it 

 (406, 444). Very strong light, however, makes it wander 

 about until it happens to reach a more shaded region. 

 Thus if the animal is subjected to light either above or 

 below a certain "optimum," it is restless. A "vague uneasi- 

 ness" is the kind of psychic accompaniment to this behavior 

 most naturally suggested ; repeated strong mechanical stimu- 

 lation will also make the animal wander about. Nothing 

 points to the existence of a visual quality. Blue and green 

 light are more frequented by Hydra than red and yellow 

 light; this parallels the effect of colored rays on Amoeba 

 (444). Widely distributed through the animal kingdom is a 

 kind of equivalence, for reaction purposes, between blue or 

 violet and white light on the one hand, red light and dark- 

 ness on the other. 

 i 



On the hydroid colonies of Tubularia no change of light 

 intensity operated as a stimulus (319). In Actinians the 

 only evidence that the reactions due to light differ from those 

 otherwise produced lies in the greater slowness of the former. 

 Many sea-anemones are wholly unaffected by photic stimu- 

 lation, Sagartia lucia and Metridium, for example (160). 

 Many others have been found to contract when illuminated 

 (150, 207, 291). Eloactis producta expands its tentacles only 

 in light of low intensity, taking about fifteen minutes to do so 

 when covered with a hood, and retracting in five minutes 

 when the light is restored. This retraction is decidedly 

 slower than that produced by mechanical stimulation (160). 

 That the responses to light are more marked in animals which 

 have been living in comparative darkness than in those 



