420 PROCEEDINGS OF THE AMERICAN ACADEMY. 



single eye comparable to the eye of a planarian ; and to these Joseph 

 (: 04) added certain large cells in the anterior part of the tube which, 

 from their structure, he believed also to be light-receptors. 



To ascertain what part of the body of amphioxus is sensitive to light, 

 I had planned to use local stimulation, and with this in view I arranged 

 an acetylin light with a condensing lens and a pinhole diaphragm, so 

 that I could have at command a small beam of strong light with which 

 to test locally the various parts of the animal's body. Unfortunately 

 the strongest artificial light that I could get was insufficient to call 

 forth an invariable reaction, and I was at last driven to use con- 

 centrated sunlight for this purpose. This was obtained by mounting 

 a mirror in an open space adjacent to the laboratory, and so directing 

 it that a horizontal beam of sunlight was thrown through a window 

 into the laboratory. This beam of light was screened of its heat by 

 being made to pass through seven centimeters of water contained in a 

 glass vessel with flat sides, and it was concentrated by a large lens 

 whose principal focus was about twenty-five centimeters. A few centi- 

 meters nearer the lens than its principal focus and in the cone of con- 

 centrating light, an iron diaphragm with a pinhole was placed that 

 intercepted all the light except that which passed through the pin- 

 hole. In this way a well-circumscribed minute beam of intense light 

 was obtained, and by means of this beam the body of the amphioxus 

 was explored while it rested in a glass dish of sea water with flat sides. 

 It was found by experiment that the dish containing the amphioxus 

 could be moved about with considerable freedom without disturbing 

 the animal. In this way the beam of light was brought to bear on any 

 desired part of the animal's body. 



My first experiments were directed toward ascertaining the value of 

 the so-called eye-spot at the anterior end of the nerve-tube as a recep- 

 tive organ for light. Experiments had already been made on this 

 organ by Nagel ('94^, p. 811 ; '96, pp. 40, 80), who recorded that after 

 the animal's anterior end, including the eye-spot, had been cut off, 

 the lancelet was found to be as sensitive to light as ever, a condition 

 confirmed by Hesse ('98i\ p. 461). I repeated this experiment on 

 six lancelets. All were first tested with light and found to respond 

 when suddenly illuminated. The anterior tip of the body with the eye- 

 spot was then cut off, and after an hour all were tested again. I was 

 unable to distinguish in this second test that the lancelets were any 

 less sensitive to light than before the removal of the eye-spot, and my 

 results thus confirm those of Nagel and Hesse. 



Although these results demonstrate conclusively that the so-called 

 eye-spot is not essential to the light reactions of amphioxus, they do 



