PARKER. — THE SENSORY REACTIONS OF AMPHIOXUS. 425 



amphioxus the posterior part is much less reactive to light than the 

 anterior, and Hesse ('98 b , p. 462), who repeated these experiments, 

 could get only a trembling response to light from the anterior half and 

 no response at all from the posterior one. My own results agree ex- 

 actly with those of Hesse. I tested six fresh animals with strong sun- 

 light, and, having found them sensitive to it, I cut each one transversely 

 in two. After an hour, and again after two hours, I tested them with 

 strong sunlight : the anterier halves always trembled markedly, but I 

 could perceive no reaction at all to light from the posterior halves. 

 When, however, I touched the posterior halves with very dilute nitric 

 acid in sea water, they sprang and wriggled forward through the water 

 most energetically, showing that they were still capable of active re- 

 sponse. I am therefore convinced that cutting the animal in two has 

 a profound effect upon its powers of reaction to light, greatly dimin- 

 ishing this capacity in the anterior half and practically nullifying it in 

 the posterior half. 



Although amphioxus reacts to light thrown upon almost any part 

 of its body except the anterior end, its reactions are characteristically 

 different in accordance with the region stimulated. When light is ap- 

 plied to the sensitive anterior fourth of the body, amphioxus almost 

 invariably gives a vigorous backward spring, often accompanied with 

 backward swimming. If light is applied to the less sensitive middle 

 portion of the body, there is usually a slight backward spring, but 

 sometimes the animal simply curls the body slightly. If the light is 

 applied to the most posterior fourth, the animal almost invariably 

 springs forward. In extreme cases, at least, the resulting movement is 

 the most effective one for removing the animal from the source of 

 stimulation. This is still more clearly seen when a beam of strong 

 light parallel with the longitudinal axis of the amphioxus is directed 

 against its anterior or its posterior end. In the former case the animal 

 darts backward, and in the latter forward ; in each instance it moves 

 away from the source of light. For animals generally backward swim- 

 ming is unusual, since the majority of negatively phototropic animals 

 when illuminated from in front first orient by turning the anterior end 

 away from the light before they begin active locomotion, whereas in 

 amphioxus the locomotion is executed without the initial step of ori- 

 entation. The case is parallel to that of a positively phototropic 

 pycnogonid described by Cole (: 01, p. 201) ; this animal moves toward 

 the source of light either with the anterior or the posterior end first. 

 In the pycnogonid, however, the two kinds of movement are associated 

 with somewhat different types of locomotion, for the animal swims 

 backward toward the light or creeps forward toward it, whereas in 

 amphioxus the reaction in both cases is simply swimming. 



