PARKER. — THE SENSORY REACTIONS OF AMPHIOXUS. 439 



tail, but not even pure fresh water stimulated the middle of the animal. 

 When any of these stimulating mixtures were applied to the head, 

 the animal swam backward ; when they were applied to the tail, the 

 locomotion was forward. 



These experiments show that the surface of amphioxus is stimu- 

 lated by solutions of nitric acid (sour), potassic hydrate (alkaline), 

 picric acid (bitter), and alcohol, and by strong ether, chloroform, 

 turpentine, etc. It is also stimulated by sea water diluted with fresh 

 water, a mixture of which may prove fatal. Such stimuli were most 

 effective at the anterior end of the animal, less so at the tail, and 

 least of all at the middle, and the reactions were always such as to 

 enable the animal to avoid the stimulus. So far as these tests go, 

 amphioxus may be said to be uniformly negatively chemotropic. 



6. Interrelation of Sensory Mechanisms in Amphioxus. 



The distribution of sensitiveness of amphioxus to the stimuli dis- 

 cussed in the preceding sections follows a very simple plan. To light, 

 heat, mechanical and chemical stimuli, the anterior portion of amphi- 

 oxus is more sensitive than the tail, and the tail is more sensitive than 

 the middle region of the trunk. A more accurate comparison of the 

 distribution of sensitiveness has shown that a response to light cannot 

 be elicited when the most anterior part of the body is illuminated, 

 though this region is very easily stimulated by either heat, mechanical 

 or chemical means. This fact and the agreement of the degrees of sen- 

 sitiveness to light with the numbers of eye-cups in different parts of 

 the nerve-tube have been given a reason for the conclusion that the 

 light receptors in amphioxus are the eye-cups themselves and not the 

 nerve terminals in the skin. Since the receptors for heat, mechanical 

 and chemical stimuli, lie in the skin, they must be distinct from the 

 photoreceptors. Further evidence of this separateness is, however, 

 seen in results obtained by exhaustion. If the tail of an amphioxus is 

 stimulated by concentrated sunlight ten or twelve times, the animal 

 will reach a state in which it no' longer responds to the illumination. 

 Wbile in this state it will react, however, with great certainty when its 

 tail is stimulated by water as 37° C, by contact with a camel's-hair 

 brush, or by a f^ solution of nitric acid. Thus from the standpoint 

 of exhaustion the receptors for light can be shown to be physiologic- 

 ally distinct from those for the other stimuli. 



The extent to which separate receptors in the skin might be distin- 

 guished for the several effective stimuli cannot be judged by the distri- 

 bution of sensitiveness for these stimuli, because, so far as I could make 



