Sensory Discrimination: Vision 139 



by the right-side-up posture, where food would have to be 

 carried downward against the upward current occasioned 

 by the sinking of the animal. Light is not the only factor 

 in producing the inversion at the surface, however, for it 

 will occur in darkness. When swimming, Gonionemus 

 moves toward the light if the latter is fairly intense, but 

 comes to rest in the shaded portions of the vessel con- 

 taining it. The reaction time to light is much slower than 

 that to other stimuli, but the animal responds most promptly 

 when certain pigmented bodies at the base of the tentacles 

 are exposed to the stimulus. If the margin of the bell con- 

 taining these bodies is cut off, no reaction to light can be 

 obtained (802, 809, 825). A great variety of structures 

 apparently sensory in function is found on the bell margin 

 of different genera and species of Medusae. Some of them 

 are statocysts. Others suggest a visual function, and in 

 the Cubomedusae there are fairly well developed eyes. 



Various annelids show response to changes in light in- 

 tensity, the leech Clepsine, for example : the slightest 

 shadow cast on the surface of the water in a dish where 

 these animals are resting quietly will cause them to reach 

 up and sway from side to side in an apparent search for 

 prey (785). On the other hand Gee (256) says of the leech 

 Dina microstoma that the casting of a shadow on it makes 

 it contract. This is apparently the more primitive and 

 the more common type of response to a change in light 

 intensity. Dina contracts in Just the same way when me- 

 chanically jarred, but a difference in the physiological pro- 

 cess^involved is indicated by the fact that these leeches 

 get used to repeated shadows, and cease to respond, much 

 more quickly than they get used to repeated jars. When 

 the earthworm has partially emerged from its burrow, 

 and has iis tail still inserted, a flash of light will produce 



