INVERTEBRATE PHOTORECEPTORS 623 



mum angular size of object which may be followed or used as a visual cue 

 of movement is a measure of the acuity of the system — -the resolving 

 power of the mosaic of separate trains of impulses reaching the central 

 nervous system. The accuracy with which an organism can respond to 

 a definite shape of object in its environment is a measure of its appreci- 

 ation of and reaction to certain forms and outUnes and may demonstrate 

 estimation of distance as well as of direction. 



In photosensory mechanisms that are particularly highly developed in 

 relation to daytime activities, the ability to distinguish between different 

 parts of the light spectrum may be present — true color vision as con- 

 trasted with mere differentiation in terms of brightness. Finally, in the 

 color-discriminating and non-color-discriminating eye, as recently as 1950 

 an ability has been demonstrated whereby the plane of polarization of sky 

 light is evaluated and used by the organism in orienting its movements. 



The structural details in the invertebrate phyla follow no single broad 

 phylogenetic sequence. Analogous mechanisms have developed in group 

 after group. Homologies may be traced within classes. With so very 

 many types of body architecture, the range of photosensory mechanisms 

 might be expected to exceed its actual limits. The largest body of infor- 

 mation available concerns the compound eyes of crustaceans and insects, 

 but the widely scattered literature includes data also on a wide variety of 

 other light-sensitive structures. 



Since light sensitivity implies radiant energy trapped and the photo- 

 chemical bleaching of visual pigments is familiar among the vertebrates, 

 a search has been made for corresponding materials among invertebrate 

 receptors. Hensen (1878) reported a pale purplish pigment, bleaching 

 on exposure to light, in the eyes of the scallop Pecten. Krukenberg 

 (1882) extracted seemingly similar materials from the cephalopod retina 

 and from the eyes of various slugs and snails and presented spectral- 

 absorption curves for these extracts and for others from earthworm, 

 crayfish, cockroach, and water beetle (Hydrophilus). Most of these 

 claims have not been supported by further work. 



The cephalopod eye, being much larger and of a camera type, like the 

 eyes of vertebrates, has offered material for investigation of photosensi- 

 tive pigments. Hess (1902) reported a change in color of the squid retina 

 when dark-adapted specimens were brought into light; he cited similar 

 but less obvious changes in the cephalopods Sepia and Eledone. Escher- 

 Derivieres et al. (1938) obtained spectral-absorption curves for the retinal 

 pigment of Eledone. The pigment seemed similar in action to rhodopsin 

 in the vertebrate eye (Wald's data, 1941), and no chemical difference was 

 detected between the retinal pigments of Eledone and Octopus. Wald 

 noted that the deep purple color of the squid retina was photostable and 

 suggested that it might be a melanoid. He tested these retinas for vita- 

 min Al and for retinenci and concluded that, since the vitamin content 



