I ID PROCEEDINGS OF Till- AMERICAS ACADEMY. 



out, this distribution was the same for all such stimuli. Evidence on 

 this point was to be had, however, from the following experiments on 

 exhaustion. After about twenty applications of a fj solution of 

 nitric acid to the tail of an amphioxus, the animal usually ceased to 

 respond to this stimulus. But on testing the same part of its body 

 with water at 37° C. or with contact from a camel's-hair brush, it was 

 found to be immediately responsive. In a similar way about thirty 

 vigorous strokes of a camel's-hair brush were needed on the tail of an 

 amphioxus before it ceased to react to this form of stimulation, where- 

 upOD it was found still to be sensitive to water at 37° C. and to a solu- 

 tion of nitric acid. Finally after an animal had ceased to react to 

 water at 37 C. it was still sensitive to contact with the brush and to 

 acid. Thus, notwithstanding the fact that the distribution of sensitive- 

 ness for these several stimuli is such as to leave the question as to sep- 

 arate receptors unsettled, exhaustion shows very conclusively that their 

 operations are physiologically distinct (Parker, 07, p. 724),and as there 

 is no evidence that they may not be represented by separate terminal 

 organs in the skin, I believe that such organs are probably present. 

 To what extent a further discrimination might be possible, as, for in- 

 stance, the separation of terminal organs for cold and for heat, or for 

 the different kinds of chemical stimuli, cannot be stated, for no experi- 

 ments in this direction were undertaken. 



To all the forms of effective stimuli that I employed, amphioxus 

 responded in but one way, namely, with such movements as would 

 remove it as directly as possible from the presence of the stimulus. 

 When the stimulus was applied to the anterior end or to the middle 

 trunk region, the animal moved backward, and when the application 

 was to the tail, it moved forward. In not a single kind of stimulus did 

 the animal move regularly toward the stimulus. This negative re- 

 sponse, which seems to pervade the whole sensory activity of amphioxus, 

 is the basis of its habit of retreat and characterizes much of what it 

 does. Even feeding, which is so usually a positive operation with 

 animals, is in amphioxus a relatively passive affair and unconnected 

 with any seeking reactions. It therefore seems that the whole sensory 

 system of amphioxus is employed as the initial mechanism in removing 

 the animal from possible danger rather than as an apparatus for Leading 

 it successfully into new territory. This feature, as Steiner ('88, p. |-j) 

 has already remarked, is perhaps the most striking peculiarity of the 

 sensory reactions of amphioxus. 



The negative response of amphioxus to stimulation is of importance 

 in considering the question of the direction in which it swims. Rice 

 ('80, p. 8) declares that amphioxus always swims with its anterior end 



