INVERTEBRATE PHOTORECEPTORS 665 



from a number of angles, of which that employing electrical measure- 

 ments on optic nerve fibers seems by far the most reliable (Froehlich, 

 1913a,b; Graham and HartUne, 1935; Jahn and Wulff, 1946). Most 

 difficult to evaluate are those based on photokinetic responses, since sta- 

 tistical numbers of animals and rigorous analysis have rarely been used, 

 although interpretations have been drawn freely from small samples of 

 doubtful uniformity. More valid, seemingly, are those studies based on 

 pigment migration in the leech (Janzen, 1932) and in the eyes of crusta- 

 ceans and insects. The kinetic response of many forms to ultraviolet 

 light invisible to the human eye has led, on the one hand, to hasty con- 

 clusions that fluorescence provided full explanation and, on the other, 

 to careful studies indicating that the moth Plusia, Planaria and other 

 turbellarians, and Daphnia possess eyes responding to ultraviolet light 

 itself — not by way of any fluorescent secondary effect — that they form 

 images with it, and that dermal sensitivity in many instances extends 

 also well into this part of the radiant-energy spectrum. Merker (1929, 

 1930, 1934) investigated also the transmission of the arthropod exoskele- 

 ton in the ultraviolet and measured a significant transparency as well as 

 a relatively low level of fluorescence in normal intensities of these short 

 wave lengths. The possible importance of ultraviolet vision in insects 

 was investigated by Lutz (1924, 1933a, b) in terms of flow^ers and mates; 

 Brues (1941) extended the study somewhat in terms of supposed mimicry 

 in butterflies. 



Early studies of spectral sensitivity were concerned primarily with 

 what region of the spectrum provided most or least attractive radiation 

 for orienting or photokinetic species (e.g., Graber, 1885). Thus Loeb 

 and Wasteneys (1915a, b, 1916) concluded, on the basis of observations 

 on hydroids, green flagellates, larval polychaetes, and larval barnacles, 

 that invertebrates involved two types of photosensory substances, one 

 with a maximum of absorption in the yellowish green near 534 m^t, the 

 other with a maximum in the blue near 477 m^t. These results were in 

 rough agreement with findings of T. W. Engelmann, Mast, and G. M. 

 White. Similar but less uniform findings have come from work on 

 Volvox, various green flagellates, turbellarians, the tube-building poly- 

 chaete Clitellio, various leeches, the sea urchin Psammechinus, the clams 

 Mya and Pholas, the snails Limnaea and Littorina, the squid, the octopus, 

 and the protochordates Amphioxus and Ciona. Among the arthropods 

 much of the controversy has centered around the possibility of color 

 vision as distinct from intensity discrimination based on spectral sensi- 

 tivities. The protracted arguments in print between the ophthalmologist 

 C. Hess and the zoologist von Frisch gathered adherents in both camps — 

 those following Hess in search for information showing that color vision 

 did not exist, and those supporting von Frisch with data indicating color 

 vision. Hess based his analysis primarily on demonstrating lack of a 



