168 . S. O. MAST 



the muscles on the ocular side and the latter on the abocular side 

 of the tail. 



The contraction caused by reduction in illumination as shown 

 above continues only 1 to 3 hm. This is similar to the result 

 obtained in a nerve-muscle preparation by a single induction 

 shock. The contraction caused by an increase in illumination 

 on the other hand may be so long that it is evidently more like 

 the results obtained by a series of induction shocks. A reduc- 

 tion in light, therefore, probably produces in the eye of the tad- 

 poles of Amaroucium a single impulse which passes to the 

 muscles of the tail causing a contraction of short duration in the 

 muscles on the ocular side, while an increase in light produces a 

 series of impulses which passes to the muscles of the abocular side 

 causing a contraction much longer in duration; in fact a contrac- 

 tion sinailar to a tetanus. 



These conclusions are supported in every detail by the results 

 obtained in the observations on the two free specimens mentioned 

 above, both belonging to the species pellucidum. One was 

 loosely held under the cover-glass; the other was held in an 

 indentation in the vaseline at the edge of the enclosure under the 

 cover-glass in such a way, that while it could not progress, both 

 the anterior and the posterior end of the body could move back 

 and forth laterally. We shall consider the latter specimen first. 

 This one was observed for about five minutes after it was 

 mounted, before it was subjected to changes in illumination, and 

 it was found that during this entire time the tail vibrated con- 

 tinuously and turned alternately quite regularly to the right 

 and to the left, resulting in a change in the position of the body 

 (fig. 6, A, B). The preparat'on was now shaded by passing the 

 hand down in front of the stage and the tail at once turned 

 toward the ocular side. This forced the posterior end of the 

 body in the opposite direction against the edge of the indenta- 

 tion in the vaseline. Here it remained. The tail, however, 

 continued to vibrate and it turned from side to side at intervals 

 of approximately 2 hm. The hand was now raised; the tail 

 immediately turned sharply toward the abocular side forcing 

 the posterior end of the body toward the ocular side against the 



