262 W- J- Crozier, 



as far from the window as the walls of the aquarium would permit, 

 they frequently continued crawling up the vertical side of the vessel 

 until the water level was reached. They might remain there if the 

 light were not too intense, but in stronger light they continued to 

 move slowly around the aquarium with the anterior end toward 

 the light, until they came to rest in some shaded area. When the 

 remainder of the aquarium was in bright light a shaded portion 

 thus gradually entrapped all the individuals present. If the anterior 

 part of the animal were shaded when the body was first illuminated, 

 the holothurians moved into this shadow. 



Before considering the general effects of light on Holothuria it 

 was necessary to know the distribution of photic sensitivity over the 

 body. Using a small spot light produced by stopping down a 

 pocket flash-light, or condensing sunlight with a lens - diaphram 

 system, I explored the surface of some fifty or more individuals. 

 The experiments were made in a dark room, and, especially when 

 sunlight was used, the condensed beam was passed through 

 7—10 cms. of sea- water before striking the animals, though previous 

 experments had shown that heat did not need to be seriously con- 

 sidered as a stimulating agency. A beam of light thrown on the 

 tentacles caused them to contract slowly and one at a time as they 

 were successively ; stimulated. The tentacles again expanded, but 

 after a period of contraction of quite variable duration. If such 

 stimuli were applied one after another as soon as the tentacles 

 were reexpanded, no further retraction could be induced after the 

 öth or 6th application. 



The pedicels and papillae also collapsed slightly when illumi- 

 nated by the spot light, and continuous intense light (3 — 5 minutes) 

 caused attached pedicels to become free and to wave about slowly, 

 as in the starfish (Jennings, 1907). The contraction of the papillae 

 was slight, usually going only to the first stage (cf. page 251). Con- 

 traction of the anterior end (with or without the tentacles expanded), 

 of the posterior end, and of practically every point on the body 

 surface were produced by localized light. The part stimulated gave 

 the usual "sinking-in" reaction as in the case of mechanical stimu- 

 lation, the depth and extent of the contraction depending on the 

 intensity of the light used. 



Animals from which the anterior end (1 cm +) had been 

 removed, as well as the removed tentacle - bearing pieces, gave 

 reactions of the same kind, but more slowly than normal animals; 



