pakker. — the sensory reactions of amphioxus. 431 



4. Mechanical Stimulation. 



As has been pointed out already, the apparently great sensitiveness 

 of amphioxus to light is really sensitiveness to mechanical stimulation, 

 a form of sensitiveness long ago remarked by Merkel ('80, p. 7), who 

 observed that a vigorous amphioxus would respond by very active 

 locomotion to the lightest touch of the forceps. 



To test the reactions of amphioxus to mechanical stimulation I 

 first used a course pig-bristle mounted so that the rounded end could 

 be brought into contact with any part of the animal's exterior. When 

 the anterior end of an amphioxus resting in a shallow dish of sea water 

 was touched even lightly with the bristle, the animal usually sprang 

 backward, though occasionally forward. The backward spring was 

 often accompanied by a somersault-like movement, whereby the animal 

 became turned end for end. When the stimulus was applied to the 

 posterior part of the body, the result was almost invariably a forward 

 leap. This portion of the body, though sensitive, was not so much so 

 as the anterior end. The middle of the body was much less sensitive 

 than either of the ends, and when the tip of the bristle was applied to 

 it, there was often no reaction. When, however, a reaction did occur, 

 it was almost always a backward leap. 



In general the reactions of amphioxus to mechanical stimulation 

 resemble in essential respects their reactions to light, showing that 

 the anterior end of the animal is most sensitive to such stimuli, the 

 posterior end less so, and the middle of the body least, and that back- 

 ward locomotion usually results from stimuli applied at the anterior 

 end or the middle, and forward locomotion from stimuli at the 

 posterior end. 



By means of local stimulation the sensitiveness of different portions 

 of the body could be roughly determined. At the anterior end, though 

 the rostrum can be stimulated, the most sensitive parts are the oral 

 hood and the buccal cirri. When any of these parts is touched, back- 

 ward locomotion almost invariably follows. If the hood, but especially 

 the cirri, are touched only very lightly, they close and open with a 

 sudden movement not unlike winking. In resting animals this is 

 often carried out in what seems to be a spontaneous manner, but 

 close inspection shows that it is dependent upon the accumulation 

 on the cirri of debris from the current of water usually passing in at 

 the anterior end. When the cirri become fairly covered with minute 

 particles of coral sand, etc., this winking movement loosens these 

 particles, and at the same time vigorously expels the water from just 

 within the anterior opening of the animal, and thus removes the ac- 



