418 PROCEEDINGS OF THE AMERICAN ACADEMY. 



illuminated dish by dish in turn with the sauie lamp that had caused 

 the whole assembly of lancelets to swim wildly about when together, 

 and noted the individual reactions. Of the twenty animals tested, 

 twelve reacted, some more, some less, but none vigorously ; eight ab- 

 solutely failed to give any response whatsoever, even after continued 

 illumination. The twenty animals were then placed together in a 

 single glass dish, and, after about an hour, they were suddenly sub- 

 jected to bright illumination, with the result that they exhibited the 

 same commotion as was seen in the first of these experiments. I 

 therefore conclude that the wild swimming recorded by Willey, Nagel, 

 and Hesse is not, as they believed, evidence of great sensitiveness to 

 light, but is the result of the mechanical stimulation of one amphioxus 

 touching another, and that amphioxus, as stated by Niisslin, is really 

 only very slightly sensitive to light. 



Rohon's belief that the so-called light reactions of amphioxus are 

 really reactions to radiant heat is not supported by my observations. 

 Contrary to the statements of Rohon, amphioxus is responsive to 

 light that has passed through a heat screen ; nor does Rohon seem 

 to have been aware of the fact, pointed out later by Krause ('97, 

 p. 514), that a few centimeters of sea water is as effective a heat screen 

 as the alum solution that he used, and that consequently in all his 

 experiments that were carried on with some depth of sea water, the 

 animals that were supposed to be subjected to radiant heat were as 

 a matter of fact as completely shielded from it as though they were 

 behind an alum screen. Kohl's concurrence in Rohon's opinion does 

 not seem to be founded on any observations of his own, for he ('90, 

 p. 182) states that he had no opportunity to work with living material. 

 I therefore believe that the slight initial locomotor response that am- 

 phioxus usually makes when a beam of light is suddenly thrown on 

 it is dependent upon the light waves themselves and not upon radiant 

 heat. 



Although amphioxus is assuredly not so sensitive to light as many 

 investigators have supposed it to be, it does show a capacity to 

 respond to a considerable range of this form of stimulus. Nagel 

 ('96, p. 80) stated that its characteristic reactions could often be 

 called forth by a relatively weak stimulus, such as the diffuse light of 

 a cloudy day. In my own experience animals that have been kept in 

 the dark for some time will usually react to light of not more than a 

 few candle-meters intensity, but the same individuals after lengthy 

 exposure to ordinary daylight will often fail to respond to a beam of 

 strong sunlight. Obviously the capacity of the animal to respond to 

 light is more or less determined by its previous condition, its sensi- 



