PARKER. — THE SENSORY REACTIONS OF AMPHIOXUS. 419 



tiveness diminishing with continual exposure to light and increasing 

 when the light is excluded from it. But even under the most favor- 

 able circumstances the reactions to light as compared with those to 

 other kinds of stimuli are relatively slight in amphioxus. 



Although amphioxus shows much diversity as to the intensity of 

 light to which it will react, in another respect its responses to this 

 form of stimulus are very uniform. In all the tests I carried out, I 

 never observed a reaction to a rapid diminution of light, and the 

 reactions to light that did occur were always the result of a rapid 

 increase of intensity. When an animal was resting quietly on its side 

 in a shaded aquarium and a beam of sunlight was suddenly thrown 

 upon it, it would usually respond by one or two vigorous locomotor 

 leaps, after which it might come to rest even in the sunlight. If now 

 the sunlight was suddenly cut off, no response followed. That this 

 failure to respond was not due to exhaustion from over-exposure to 

 light was easily shown by quickly throwing on the sunlight a second 

 time, whereupon a reaction much like the first one usually followed 

 immediately. In fact, a moderately rapid alternation of full light and 

 shadow was generally followed for a number of times by reactions to 

 the light and no reactions to the shadow till, after numerous trials, the 

 animal ceased to respond at all. Amphioxus is therefore stimulated 

 only by such rapid changes of light intensity as involve an increase in 

 the illumination. This agrees fairly well with Nagel's statement 

 ('94, p. 811 ; '96, p. 80) that sudden shadow calls forth from amphi- 

 oxus either faint responses or none at all. In my experience the latter 

 part of this statement is correct. 



Having ascertained that amphioxus is sensitive to light, the next 

 question that naturally arises is what portion of its body serves as the 

 receptive organ for this stimulus. Numerous answers have already 

 been given to this question. The conspicuous pigment spot at the 

 anterior end of the nerve-tube discovered, according to J. Miiller ('39, 

 p. 198), by Retzius, was held by the former ('44, p. 95) and many other 

 investigators to be a primitive eye. Hasse ('76, p. 287) believed that 

 the light receptors were two lateral patches of integumentary cells, 

 one on each side of the flattened anterior end of the animal. Niisslin 

 ("77, p. 25) was of opinion that the extreme anterior portion of the 

 dorsal fin was the part sensitive to light. Krause ('88, p. 136), who 

 discovered in the substance of the nerve-tube a pigment that he believed 

 resembled visual purple, was thereby led to assume that this tube was 

 the receptive organ for light. Nagel ('94% p. 811) claimed that the 

 whole outer skin was receptive to light. Hesse ('98', '98'^) maintained 

 that the numerous small pigment spots of the nerve-tube were each a 



