592 BIOLOGICAL EFFECTS OF RADIATION 



The activity of specimens without eyes depends upon the intensity of 

 the hght in which they are exposed. These responses are correlated 

 with the amount of Ught energy received, rather than with the rate of 

 change in the amount received. There are, therefore, two kinds of 

 response in Planaria, one dependent upon the eyes, the other not. There 

 is nothing known concerning the chemical and physical changes associated 

 with either of these responses or with the stimuli which induce them, and 

 practically nothing concerning the quantitative relation between the 

 stimuli and the responses. 



The distribution in the spectrum of stimulating efSciency, in these 

 forms, has not been adequately investigated. Kiihn (119) says planarians 

 are positive in light of long wave-length and negative in light of shorter 

 wave-length, and Beuther (18) maintains they have color vision, but 

 Koehler (116) thinks this has not been established. Merker and Gilbert 

 (167a) conclude that they "see" ultra-violet, but they give no information 

 concerning discrimination between different regions in the visible 

 spectrum. 



MOLLUSCS 



Response to light has been fairly intensively studied in several differ- 

 ent molluscs: Mya, Peden, Helix, Limax, and Agriolimax, 



MYA 



Mya, the long-necked clam, responds very definitely to rapid increase 

 in light by contraction of the neck, and to rapid decrease in light by 

 closing the siphons (Nagel, 175). It contains numerous photoreceptors 

 on the inner surface of the siphons. They are found throughout its 

 entire length, but they are most abundant in the central region. Their 

 distribution is closely correlated with the distribution of sensitivity, and 

 the reaction time varies directly with the number of receptors illuminated 

 (Light, 125). 



Hecht (83 to 89) investigated in its various phases, the response to 

 increase in light intensity in Mya and also in Pholas and Ciona,^ and 

 reached the following conclusions: 



The time between increase in illumination and contraction (reaction 

 time) is composed of two periods. During the first of these periods, the 

 exposure period, the organism must be in light; during the second, the 

 latent period, it does not need to be in light. In a given intensity, as 

 the exposure period increases, the reaction time and the latent period 

 decrease to a minimum and then remain constant. The minimum latent 

 period does not vary with the intensity but the exposure period t, produc- 



^ Pholas is a mollusc which is much like Mya in structure. Ciona is an ascidian 

 which is not closely related to Mya, but it contains siphons which respond to increase 

 in illumination very much like those in Mya. 



