PARKER, — THE SENSORY REACTIONS OF AMPHIOXUS. 417 



fish is indescribable," and Nagel ('9P, p. 79) states that "plotzliche 

 Belichtung liisst dann die samtlichen Exemplare wild durchs Wasser 

 jagen." Hesse ('98i\ p. 461) confirms these observations and records 

 that light calls forth vigorous swimming. On the other hand, Niisslin 

 ('77, p. 23), who also tried sudden illumination, affirms that amphioxus 

 is only very slightly sensitive to light, and Rohon ('82, p. 38) ex- 

 presses the belief based on experimental evidence that the so-called 

 light reactions of this animal are really reactions to heat, and that it 

 is not sensitive to light at all, or at most only to a very slight degree, 

 — an opinion concurred in by Kohl ('90, p. 185). 



In consequence of this difference of opinion the first question to be 

 settled was, whether amphioxus was or was not sensitive to light. I 

 therefore repeated the experiments made by Willey, Nagel, and Hesse, 

 and with confirmatory results. When sunlight, daylight, lamplight, 

 or even candle-light was allowed to fall into a previously darkened 

 glass dish containing a dozen or more amphioxus, the whole company 

 swam about for a minute or so in wild confusion and then dropped as 

 though exhausted to the bottom. At first sight this seemed to be 

 conclusive evidence of the great sensitiveness of amphioxus to light, 

 but a more careful scrutiny of the steps in the experiment showed that 

 this was not necessarily so. When light first fell upon the dish, all 

 the lancelets did not begin at once to swim about excitedly. What 

 usually happened was that a few moved slightly, and in doing so they 

 touched others ; these then sprang suddenly into active locomotion, 

 and in an instant the whole assembly was swimming in wild confusion. 

 Thus it would seem that, while light was the initial stimulus for a 

 few individuals, the wild and excited swimming which gave the im- 

 pression of great sensitiveness to light was not due directly to this 

 factor, but to mechanical stimulation caused by mutual contact. 



To test this h}^othesis I placed a shallow dish of sea water con- 

 taining twenty live amphioxus in a dark room and, after about an 

 hour, I threw upon it the light of a strong lamp ; in a few seconds all 

 the animals were swimming as though in the utmost excitement. I 

 then let them rest in the dark for a full hour, whereupon, without 

 illuminating the dish, I felt for one with a glass rod, and, having 

 touched it, I soon heard an agitated movement in the dish such as 

 had followed the previous sudden illumination. Upon turning on 

 the light the animals were found to be in as much commotion as at 

 the trial in which light had been the initial stimulus. I then took the 

 twenty animals that had been used in these two experiments and put 

 each one in a separate dish of sea water and placed each dish in an 

 approximately light-proof compartment by itself After an hour I 



VOL. XLIII. — 27 



