THE PHOTIC SENSITIVITY OF CIONA INTESTINALIS. 



By SELIG HECHT. 



{From the Physiological Laboratory, College of Medicine, Creighton University, 



Omaha.) 



(Received for publication, September 11, 1918.) 



I. Preliminary Statement. 



The sensitivity of Ciona intestinalis to light furnishes an unusual 

 opportunity for analysis of the receptor process occurring in the sense 

 organ. This is because a sudden increase in illumination produces a 

 vigorous and clear cut response on the part of the animal. Such a re- 

 action to a source of stimulation, which in itself may be measured 

 accurately and controlled with comparative ease, makes the sensory 

 behavior of Ciona amenable to a strict, quantitative treatment. My 

 experiments were therefore concerned, first, with the localization of 

 the photic sense organs, and second, with variations in the magnitude 

 of the stimulating agent under conditions calculated to throw light 

 on the nature of the sensory process. 



The species used is identical with the Ciona intestinalis common in 

 the Mediterranean Sea and along the European coast (Ritter and 

 Forsyth, 1917). At San Diego, Cahfornia, it is to be found in enor- 

 mous numbers attached to piles, and forming thick carpets on the 

 under side of floats. In fact, the hundreds of individuals employed 

 were all taken from a single float in San Diego Bay.^ 



II. Photic Sense Organs. 



1. Ciona possesses two qualitatively different means of reacting to 

 light. In one case the response is a general one, involving the or- 

 ganism as a whole. In the other the activity is confined to the lo- 



1 My thanks are due to the Scripps Institution for enabling me to come to the 

 station at La Jolla, and to the director, Dr. Wm. E. Ritter, and his staff for the 

 many courtesies extended to me during my stay. 



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