PHOTIC REACTIONS OF TADPOLES 275 



such alternative photochemical systems have been employed by 

 Hecht ('18, '19a, b, 1920) in a discussion of the nature of the 

 photosensitivity of Ciona and Mya. 



2. The jfliotoreceptors ' 



The isolation of skin photoreceptors for histological examina- 

 tion has met with difficulty by all workers who have undertaken 

 it. No progress has been made other than regional localization 

 of such organs. It is not difficult, however, to imagine a struc- 

 tural condition for photoreception such as described by Polara 

 ('06). In Holothuria poli he describes pigment cells in close 

 association with nerve terminations. These pigment cells, 

 he suggests, are a part of a sensory apparatus for the reception 

 of light. As a result of continuous illumination, the pigment 

 cells are decolorized and associated with this process, there is 

 a loss in the photosensitivity of the animals. In the darkness 

 pigment reappears and with it photosensitivity is regained. 

 Similar observations have been made by Crozier ('14) in Holo- 

 thuria surinamensis and H. captiva. A fluorescent pigment, 

 confined to the superficial layer of the integument, showed ab- 

 sorption in the blue-green band of the spectrum, thus suggesting 

 that the process of decolorization is of a chemical nature. 



The photoreceptors in the tadpoles must be so situated in the 

 integument as to be partly or entirely concealed when the 

 melanophores are in an expanded condition. Tadpoles dark 

 in appearance either did not respond when light was applied or 

 showed a reaction-time totally different from tadpoles light in 

 appearance. This behavior was studied in detail and with 

 considerable care. Before the facts of this situation were known, 

 much difficulty was experienced in the experimental work. For 

 this reason, after certain preliminary trials, only tadpoles of the 

 same degree of coloration were used. 



Photoreceptors in animals are analj^zers for appreciating 

 changes in light and darkness. When the animals are living in 

 their natural habitats this type of sense organs must play an 

 appreciable role. In species which respond only to a decrease of 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 34, NO. 2 



