CHAPTER XXVI 



Photosensitivity in invertebrates' 



LOR us J. MILNE2 

 MARGERY M I L N E^ 



Durham, .h'eiv Hampshire 



CHAPTER CONTENTS 



Photosensitivity in Unicellular Organisms 

 Cells Without Obvious Photoreceptors 

 Cells With Obvious Photoreceptors 

 Photosensitivity in Multicellular Organisms 



Photosensitivity Mediated Without Obvious Receptors 



Ganglionic photosensitivity 



Peripheral photosensitivity 

 Photosensitivity Mediated Through Unicellular Eyespots 

 Photosensitivity Mediated Through Multicellular Eyes 



Compound eyespots 



Ocelli or simple eyes 



Compound ocelli or aggregate eyes 



Stemmata 



Compound eyes 



Camera-style eyes in moliusks 



Camera-style eyes in annelids 



Camera-style eyes in arthropods 

 Phenomena Related to Stimulus Intensity 

 Pigment Migration Within the Eye 

 Spectral Sensitivity and Color Vision 

 Form Perception and Pattern Recognition 



EYE-MINDED MAN IS proDC to forgct that the fundamen- 

 tal irritability of protoplasm includes a sensitivity to 

 radiant energy in the spectral region he knows as 

 light. An eye is a specialization with which a multi- 



' Contribution from the Scripps Institution of Oceanography, 

 La Jolla, California, New Series No. 967. The information in 

 this chapter has been assembled with the aid of research grants 

 from the .American Academy of .\rts and Sciences, the Ameri- 

 can Philosophical Society, the E.xplorers Club and the Society 

 of the Sigma Xi. 



' Professor of Zoology, University of New Hampshire. 



^ Recently Visiting Professor of Biology, Northeastern Uni- 

 versity, Boston, Mass. 



cellular animal may gain additional information from 

 a light stimulus. Usually it is a device allowing a 

 central nervous system to be better informed about 

 events in the surrounding environment. An eye im- 

 plies a nervous mechanism of some kind, but the 

 converse is not true. 



In photosensitivity the initial event is absorption 

 of photons — the quanta of radiant energy — by some 

 substance which is altered by this addition. When 

 compared to thermal reactions, most photochemical 

 changes appear to be in a class by themselves charac- 

 terized by temperature coefficients so small that they 

 are described as 'temperature-independent.' Through 

 the temperature range within which living things are 

 active this is correct enough. 



Since only the absorbed energy is effective in pro- 

 ducing a photochemical change, every photosensitive 

 mechanism must contain a chemical substance which 

 can trap photons. When the concentration of such a 

 substance is high enough so that a few per cent of 

 incident photons are absorlied, we may detect the 

 absorption as an opacity and recognize the absorbing 

 substance as a pigment. 



So far none of the pigments found to be responsible 

 for photosen.sitivity in living systems are neutral in 

 their absorption. They are not gray but colored be- 

 cause they absorb most at one wavelength and less 

 at others. This feature determines the spectral sensi- 

 tivity of the system. 



For an animal to be informed continuously regard- 

 ing the radiant energy reaching its surface, it must 

 be able to produce continuously the pigment which 

 is altered by the absorption of photons. In the dark 

 this production would be expected to decrease in 

 rate until the pigment reached a maximum concen- 



621 



