PARKER. — THE SENSORY REACTIONS OF AMPHIOXUS. 421 



not show that this spot may not be a Hght-receptive organ. To test 

 this possibility I attempted by means of the minute beam of light 

 already described to illuminate the spot exclusively, and to see if a 

 reaction resulted. This was by no means easily done, for the spot is 

 so small that its position in the living animal cannot well be observed 

 directly, but must be surmised. Furthermore, when the light enters 

 the substance of the animal, it becomes much scattered, and hence 

 may reach other parts than those it is intended to illuminate. Never- 

 theless, it was possible on a number of animals to throw intense light 

 on the eye-spot without getting a response, though, when the light was 

 moved to a position somewhat posterior to the spot in question, a 

 vigorous response followed. I therefore conclude that not only is the 

 so-called eye-spot of amphioxus unessential to its light reactions, but 

 that this organ is in no sense a light-receptor. These physiological 

 results, then, support the view long ago advanced by Stieda('73, p. 51) 

 on the basis of anatomical evidence, that this spot is not a visual organ. 

 For this reason I shall in future call it simply the anterior pigment 

 spot, though its nervous nature seems well established by the recent 

 work of Edinger (:06). In a similar way I tried to get reactions from 

 lancelets by directing the beam of light on the flattened sides of their 

 anterior ends, where, according to Hasse ('76), light-receptive organs 

 were supposed to be located. In no instance did I get a reaction, and 

 I therefore agree with Niisslin ('77, p. 12) and with Kohl ('90, p. 183) 

 in denying the existence of light-receiving organs in this region. 



Lancelets from which the anterior end of the dorsal fin had been 

 removed were as sensitive to light as before the removal, nor did 

 normal lancelets react to the small beam of light when it was thrown 

 on this part of the fin. I therefore believe that Niisslin ('77, p. 25) 

 was in error when he declared that the anterior end of the dorsal fin 

 was the portion of the animal that was sensitive to light. 



The part of the body of amphioxus that can be stimulated by light 

 extends from a point a little behind the anterior end posteriorly to 

 the tip of the tail. A beam of concentrated sunlight thrown across 

 the body in any region between these two points always elicits some 

 response. Krause ('97, p. 514) states that the anterior end somewhat 

 distal ^ to the anterior pigment spot is most sensitive to light, and 

 that the tail end is not sensitive at all. My results, as already stated, 

 are almost precisely the reverse of these. I have found the anterior 

 end, both in front of the anterior pigment spot and at least immediately 

 posterior to it, insensitive to light, and the tail end extremely sensi- 



* By distal Krause means, judging from the context, posterior. 



