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PHOTIC SENSITIVITY OF CIONA INTESTINALIS 



eral body sensitivity could not therefore account for the retraction 

 reflex. By exploring the body with a small beam of Hght, I found that 

 the seat of this photic sensitivity was in the small area occupied by the 

 intersiphonal ganglion and its related structures. When light was 

 focussed on this spot alone the reaction given by Ciona was in all 

 particulars the same as when the animal as a whole was illuminated. 

 No other region of the body possessed this property. As an example 

 of many similar experiments the record of Animal 30.5 is given. 



It will be noticed that the reaction time to stimulation of the 

 ganglionic region is less than for the exposure of the entire body. 

 This is because the light used for spotting the ganghon region was con- 

 centrated to a small area by means of a lens. It will be shown pres- 

 ently that the reaction time varies inversely as the intensity of the 

 light. 



It is difficult to say which of the several organs composing the 

 neural mass contains the photoreceptors. Experiments with the 

 opaque Ascidia atra of Bermuda (Hecht, 1918, h) have shown that its 

 photic sensitivity may be demonstrated only by stimulating the dorsal 

 wall of the inside of the oral siphon below the ring of tentacles. This 

 corresponds to the position of the dorsal tubercle, a structure which, 

 in Ciona, is closely affiliated with the ganglion. However, though I 

 performed many operations involving the removal of the entire neu- 

 ral mass, as well as only portions of it, the results were of rather 

 doubtful interpretation. 



The difficulty probably Hes in the fact that a disturbance in the 

 integrity of the neural complex interferes with the conduction of im- 



