360 J. M. D. OLMSTED 



ahead," "The lack of orientation in moving from a source of 

 light is much more striking in the Holothurians," and "there 

 is no orientation in these animals" — hold for species like Thyone 

 briareus (Peare, '08), but not for Holothuria surinamensis 

 (Crozier, 14 a), Holothuria captiva (Crozier, '14 b), or Synap- 

 tula hydriformis. 



If the heads of synaptulas are cut off just back of the ten- 

 tacles, they, too, will move to the side of the dish away from 

 the source of light, whether it be the sun (direct or indirect 

 light), or an electric (32 candle power) lamp. When the pos- 

 terior portions of these headless animals are placed with their 

 anterior ends in the sunlight, they also show response to the 

 light by bending their cut ends away from the light, so that the 

 body becomes bent into a U. If, however, these posterior 

 portions are placed some distance (five feet) from a window 

 on the side of the room opposite that where the sun is shining 

 in, or near an electric lamp, they do not orient to the light, but 

 remain practically as they were placed. If the tentacles only 

 are cut off (the eye-spots being left intact), the animals bend 

 their anterior ends away from the light, and gradually bring 

 their posterior ends around until the animal lies in a straight 

 line headed away from the source of light. In this respect 

 they differ from the headless synaptulas, since the latter remain 

 bent. Posterior headless pieces placed in the dark with their 

 cut ends arranged in a line, were found after 30 minutes to be 

 in practically the same position as that in which they had been 

 placed at the beginning of the experiment ; therefore the bend- 

 ing away from the light of the headless portions is a true photic 

 response, and not the result of injury. 



Spots of light thrown on different parts of the body of 

 Synaptula showed that all parts were sensitive to light. Experi- 

 ments were first tried in which the synaptulas were placed 

 on the bottom of a dish containing 20 cm. of water. A spot 

 of sunlight, concentrated by means of a lens of 25 cm. focal 

 length, was thrown on the various parts of their bodies. 

 The response, no matter what portion of the body was illumi- 

 nated, was so violent, that it seemed almost impossible that it 



