PHOTIC SENSITIVITY OF BALANOGLOSSUS 213 



fall upon the Ptychoderas. They immediately gave evidence of 

 increased peristaltic activity, and those individuals judged to 

 be in good condition soon turned the proboscis away from the 

 light, and began to creep in the same direction. When illumi- 

 nated in this way the proboscis becomes very mobile, and peristal- 

 tic waves pass rapidly along the genital pleurae, thorax and 

 abdomen. The collar also exhibited wavy motions, particularly 

 along its anterior edge. 



When Ptychodera, or Glossobalanus, is illuminated from the 

 side, it promptly turns the proboscis end toward the opposite 

 side. Neither here nor in its movements after orientation is 

 accomplished does Ptychodera make any 'mistakes'; its move- 

 ments are directed immediately away from the light. 



Fragmented animals were also employed, and it was found that 

 anterior halves, separated in the region of the genito-hepatic 

 transition, oriented in a normal manner and moved away from 

 the light. Posterior halves, and pieces of the abdomen (includ- 

 ing the caudal extremity) were stimulated to slightly increased 

 peristaltic movement, and progressed away from the light tail 

 first. Isolated posterior fragments tend to move in a caudad 

 direction, even in the dark, but otherwise show relatively little 

 in the way of organized activity until they have begun to 

 regenerate. 



WTien complete Ptychoderas, or anterior fragments, are placed 

 ventral side uppermost they continue for a time their usual peri- 

 staltic movements, but progress in a caudad direction. Animals 

 so situated, when exposed to light thrown on the anterior end, 

 contract lengthwise^to a notable extent, but continue to move 

 backward. The reversed direction of locomotion, which results 

 from the abnormal position of the balanoglossid, is a consequence 

 of its normal peristaltic motions, and of the fact that it cannot, 

 when so situated, adhere to the substratum at the anterior end. 



It may be mentioned that a Ptychodera from which the collar 

 nervous system (the delaminated part) has been removed, but 

 otherwise intact, will orient in the usual way. These specimens, 

 as well as the normal individuals, were found to burrow with the 

 proboscis turned away from the light. 



