2/8 A. S. PEARSE. 



Although such characteristic responses are given when the light 

 intensity is decreased no reaction occurs when there is a corre- 

 sponding increase. An individual will contract at once when an 

 object is interposed between it and the light, but it gives no re- 

 sponse if the object is removed after a time. Furthermore, when 

 light from a large oil lamp or from the sun was suddenly reflected 

 from a mirror on a feeding individual there was no response. 

 This sensitiveness to decrease in the light intensity and lack of 

 response to an increase is similar to the reactions observed by 

 Hargitt (:o6) in Hydroides dianthns d,nd other annelids. UexkuU 

 ('97) has also described striking shadow responses in sea-urchins. 



Thyone gives well-marked locomotor responses to light which 

 may be illustrated by the following experiment : Eight indi- 

 viduals were successively placed in a shallow rectangular glass 

 dish which measured 29 cm. long by 25 cm. wide, and contained 

 sea water. The dish was enclosed in a black box which had an 

 opening at one end. This opening was directed toward the 

 window so that light was admitted from only one direction. 

 The animals were always placed with the long axis of the body 

 at right angles to the light rays and the direction of the subse- 

 quent movement was then observed. In a series of twenty-four 

 reactions the locomotion in every case carried the animal away 

 from the light to the end of the dish, but there was no definite 

 orientation of the body in relation to the light. In ten of these 

 negative responses the anterior end was ahead as the individual 

 moved ; in nine instances the posterior end preceded the anterior ; 

 and in five the locomotion was straight toward the right or left. 

 Not one of the eight individuals moved in every case with the 

 anterior or posterior end in front. 



The influence of the negative light response is also apparent 

 in the righting reactions. When individuals were placed in the 

 same position as in the experiments described in the last para- 

 graph, except that the ventral side of the body was uppermost, 

 the righting reaction usually carried the ventral surface away 

 from the light. Two individuals were given fifteen trials each in 

 the manner just described, the anterior end being turned alter- 

 nately toward the right and left in successive trials. One of 

 them turned four times toward the light in righting itself and 



