CHAPTER 14 



Invertebrate Photoreceptors 



LoRus J. Milne, assisted by Margery J. Milne 



University oj New Hampshire, Durham 

 Photosensory structures. Summary. The phoiosensory mechanism. Note. Refer- 



ences. 



Information concerning the light receptors of invertebrate animals has 

 come from behavior studies, anatomical investigations, and a relatively 

 small number of workers who have made electrical recordings of impulses 

 in nerve fibers serving sensory mechanisms. Commonly the reactions of 

 an animal to light have led to a more complete understanding of the 

 photosensory basis for which structural features can be found. In all 

 metazoans, however, the nervous system interpreting the impulses from 

 a sensory cell is at least as important in determining behavior as are the 

 anatomical aspects of the photoreceptors themselves. 



At its simplest, an organism can discern the difference between being 

 in the hght and being in the dark. Since light and dark are relative 

 terms, it is not surprising that most such organisms can distinguish 

 between hght intensities and that many of them react visibly when the 

 intensity is altered abruptly. All of them respond differently according 

 to the distribution of energy among the wave lengths of the stimulating 

 light. This is a measure of their spectral sensitivity. 



A higher degree of organization in both sense organs and nervous sys- 

 tem allows an animal to learn the direction from which a beam of light 

 comes, and hence also to detect any change in direction. The organi- 

 zation may consist merely in differential sensitivity within the cell, or it 

 may be assisted by shadow-casting pigment masses or light-concentrating 

 lens systems. In a metazoan the ability to discriminate betAveen differ- 

 ent directions of illumination usually rests on the facts that numbers of 

 sensory structures are present, that the various units show individual 

 polarity in reacting to a light stimulus, and that the units are oriented in 

 a number of different directions. Hence a beam of light stimulates the 

 photosensory units differentially. Any alteration in the direction from 

 which light comes involves a change in the degree of stimulation of the 

 many receptors — a reduction in intensity for some, an increase in inten- 



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