44:2 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the sensitiveness of the fragment becomes much lowered, it is only 

 necessary to put the piece in very dilute picric acid to call forth the 

 characteristic locomotion again. Johnston (:05, p. 124), however, states 

 that even a small piece of the tail of amphioxus can swim well and be- 

 haves much as the whole animal does. Nagel ('94^, p. 811 ; '96, p. 79) 

 declares that both halves of an amphioxus react promptly to light, but 

 less energetically than the whole animal does. But Danilewsky ('92) 

 maintains that the halves react, at least to mechanical stimuli, very 

 differently ; the anterior half is quite sensitive to this form of stimulus, 

 but the posterior half can be brought to react only with difficulty. 

 Krause ('97, p. 514) declares that the anterior half reacts vigorously 

 to light and the posterior half only slightly. Hesse ('98^, p. 462), 

 however, states that after division the anterior part only trembles on 

 being illuminated and the posterior part gives no reaction whatever. 



My own observations on B. caribbaeum lead me to believe that 

 whether reactions will be given by both halves of this amphioxus or 

 not depends quite as much upon the nature of the stimulus as upon 

 any other factor. To light, as already stated, I have never been able 

 to get any response from the posterior half, though the anterior half 

 regularly trembled whenever strong light was thrown upon it. In 

 these respects my results agree exactly with those of Hesse, and they 

 were, moreover, so uniform and regular that I am led to suspect the 

 accuracy of Kra use's and of Nagel 's statements, at least so far as 

 they apply to the posterior half of amphioxus. After the nerve-tube 

 is cut, this part seems no longer able to respond to light. That this 

 is due to the small number of eye-cups in this region, as Hesse be- 

 lieved, is not true, for, as a matter of fact, these cups are almost as 

 numerous in the tail region as in any other part of the animal. In 

 my opinion the failure of the posterior half of amphioxus to react to 

 light is not due to the lack of sensitiveness, but to the interruption 

 of some centrally situated, reflex path. In the posterior half, appar- 

 ently, the sensory neurones that are stimulated by light cannot trans- 

 fer their impulses directly to the motor neurones of the same region, 

 but only indirectly through the anterior part of the nerve-tube ; hence 

 when this is removed the reflex ceases. It is in this way, rather than 

 through altered sensibility, that an explanation of this phenomenon 

 will, I believe, be found. 



To mechanical, and especially to chemical, stimuli I found both 

 halves of amphioxus to be responsive, not, however, as Steiner de- 

 scribes, but rather as stated by Danilewsky, in that the anterior part 

 was found to be quite sensitive and the posterior part slightly so. 

 These observations suggest that the central tracts over which photic 



