ORIENTATION OF AMPHIOXUS 39 



About thirty individuals were placed in three glass jars con- 

 taining a layer of 'amphioxus sand' and were stimulated by the 

 method just described. All responses in which the orientation was 

 doubtful and those responses which included wild dashes or ex- 

 cessive somersaulting were disregarded; of the responses recorded, 

 some were observed during the whole course and others were 

 judged chiefly bj^ the orientation at the beginning and partic- 

 ularly at the slowed-down finish of the swimming reaction. 

 A tabulation of observations obtained in this manner is as 

 follows : 



Total number of responses 50 



Anterior end in advance 41 



Posterior end in advance 9 



In a considerable number of animals the first movement was 

 backward, but the direction was quickly reversed bringing the 

 anterior end in advance; this condition will be discussed later 

 in connection with other experiments. 



When, however, Amphioxus are free to move in unlimited 

 space, the somersaulting and the quick reversals of direction 

 make accurate observations as to what is occurring during the 

 response very difficult. To obviate this difficulty a large por- 

 celain pan was partly filled with water, giving depths which 

 varied from 0.5 cm. at the edge to 1.25 cm. at the center. The 

 movements of Amphioxus under these conditions were as follows : 

 When a stimulus (a bristle or finely drawn glass rod) was applied 

 to the anterior end, the animal responded with a backward spring; 

 if locomotion was now continued, this direction was not main- 

 tained for more than a few centimeters, for a quick reversal 

 occurred, exceedingly difficult to follow, but which seemed to 

 include a doubling end for end succeeded by a partial rotation 

 of the long axis of the animal about its middle point; as a 

 result the Amphioxus swam away with its anterior end in 

 advance and usuall}^ at an acute angle with the direction toward 

 which it was heading before stimulation occurred. During 

 subsequent swimming these reversals occasionally occurred and 

 as a result the posterior end of the animal might be in advance 



