586 BIOLOGICAL EFFECTS OF RADIATION 



cate that it is correlated with the rate of metabolism, but they give no 

 information concerning the processes involved. 



The distribution of stimulating efficiency in the spectrum for Volvox 

 (Laurens and Hooker, 124) and Gonium (Mast, 144) is essentially the 

 same as it is for Euglena; but for the closely related forms Pandorina 



o 



and Spondylomorum (Mast, 144) the maximum is at 5350 A in place of 

 4850 A, and the effective region extends from this wave-length much 

 farther in either direction than it does for Euglena, Gonium, and Volvox. 



COELENTERATES 



Relatively few coelenterates respond to light and the response is 

 not marked in those that do. They were, however, among the first 

 animals without eyes investigated in reference to these responses. 

 Trembley (204), in 1744, exposed to light green hydras in a jar covered 

 with an opaque case containing an opening on one side and he found that 

 they migrated toward this opening, but he gives no information con- 

 cerning the nature of the response and the process of aggregation. 



It is now known that they ordinarily move fairly directly toward the 

 light ; that they move in the opposite direction if the light is very intense ; 

 that they tend to come to rest in darkness; and that aggregation on the 

 more highly illuminated side of a vessel is consequently not due to the 

 relation between rate of movement and light intensity. There is no 

 fixed orientation, but if they are photopositive, they tend to face in the 

 direction of the light more than in other directions and they usually move 

 only when they face in this direction. There are no definite shock- 

 reactions and nothing is known concerning the processes involved in 

 stimulation. The anterior end is probably more sensitive than the 

 posterior but no photoreceptors have been found. Response to light 

 differs radically from the response to electricity (Mast, 140). 



Blue probably has a higher stimulating efficiency than other regions 

 of the spectrum. Wilson (223) maintains that daylight, the full spec- 

 trum, has a lower stimulating effect than the blue of daylight. This 

 is important for it indicates that the action of the blue is antagonized 

 by some of the other colors in the daylight. 



The only other coelenterate which has been at all accurately studied 

 in reference to photic response is Eudendrium. Eudendrium in the 

 hydranth stage is a sessile animal. It bends directly toward the light 

 in all intensities. This response is very slow, for it requires some 48 hr. 

 to bend 90°. There is nothing in the nature of shock-reaction. Loeb 

 (126) contends that the bending is due to contraction on the more highly 

 illuminated side and that the process of orientation is identical with that 

 in plants. The evidence in support of these contentions is, however, 

 extremely weak. Growth' appears to be involved in the process of orien- 

 tation and this is doubtless related to difference in the luminous intensity 



