422 PROCEEDINGS OF THE AMERICAN ACADEMY. 



tive. As Krause in his first description of the animal ('88, pp. 132 

 135) stated that it rests with its tail out of the sand, and in his 

 later account ('97, p. 513) that the head usually projects, a fact well 

 established since the time of J. Milller ('41, p. 399), is it not possible 

 that in his study of the light reactions of this somewhat ambiguous 

 form Krause has fallen into the not unnatural error of confusing 

 the ends ? 



The extent of the region that is sensitive to light in amphioxus very 

 nearly coincides with that of the nerve tube, and evidence obtained by 

 local stimulation points to this structure as the part of the animal 

 stimulated by light. Krause ('88, p. 132; '97, p. 513) has advanced 

 the opinion that the bluish coloring matter that appears in the walls 

 of the tube when this structure is treated with alkali is similar to the 

 visual purple of the retina, and is in this way connected with the light 

 receptive function of the tube. On treatment with alkali this coloring 

 matter, according to Krause, becomes visible around the pigment spots 

 in the tube, and among these are included the anterior pigment spot as 

 well as the series of smaller spots that extend through almost the whole 

 length of the tube ; but it has just been stated that by local stimula- 

 tion the anterior pigment spot can be shown to be insensitive to light, 

 and since this coloring matter is as characteristic of that spot as of the 

 other spots in the tube, I do not believe that the blue substance de- 

 scribed by Krause has any essential connection with the light-receptive 

 apparatus. As Hesse ('98% p. 556) has pointed out, Krause's belief 

 that the blue is analogous to visual purple is unsupported by any good 

 evidence, for this material shows no such relation to light as is charac- 

 teristic of visual purple. It therefore seems to me that Krause's view 

 is untenable. 



Since amphioxus shows no response when strong light is thrown on 

 the anterior end of its nerve-tube in front of the third or fourth seg- 

 ment, a region in which occur certain large cells supposed by Joseph 

 (: 04, p. 21) to be sensitive to light, I conclude that these cells are not 

 open to that kind of stimulation and that the light-receptive organs 

 must lie posterior to this region. 



Although it is impossible, for reasons already given, to illuminate 

 amphioxus locally with great precision, the exact portion of the animal 

 that is stimulated by light can be determined with fair accuracy. 

 This portion corresponds to the region in which the nerve-tube contains 

 the small eye-cups described by Hesse. This correspondence is so pre- 

 cise that it seems very probable that these organs are the true photo- 

 receptors. It must not be forgotten, however, that, in all regions where 

 light has proved stimulating, this agent in its passage into the more or 



