INVERTEBRATE PHOTORECEPTORS 625 



Compound ocelli occur in a few arthropods and echinoderms. Solitary 

 ommatidia, or structures so similar anatomically (though perhaps not 

 homologous) as to be identified as such, are characteristic of some insects. 

 Collectively the receptor cells of a multicellular eye may be regarded 

 as the retina, and each cell as a retinula. Often a retinula is specialized 

 into a nucleus-containing basal portion and a distal segment containing 

 radiating fibrils termed sensillae. The grouped receptor cells in an omma- 

 tidium often secrete along their common boundary a refractive body, the 

 rhahdom, which extends the optical axis to the most basal part of the 

 retinulae and supposedly transmits to the photosensory cells light received 

 through the more distal dioptric system. 



PROTOZOA 



Where a definite pigment spot is present in a protozoan, the photo- 

 sensory mechanism has seemed more evident, but amebas and ciliates 

 lacking such pigmented speciaUzations react also to light. Hertel (1904), 

 investigating the effect of ultraviolet at 280 m^, concluded that the 

 responses to irradiation arose from catalysis of hydrogen peroxide for- 

 mation. Harrington and Learning (1899), using the spectral regions 

 visible to man, noted a differential effect with wave length such that 

 amebas traveled rapidly in radiation of long wave lengths (red), but 

 protoplasmic streaming was retarded, stopped, or reversed by rays toward 

 the violet end of the spectrum ; white light seemed even more effective in 

 affecting activity. Mast (1932) and Mast and Stabler (1937) concluded 

 that increased illumination increased the elastic strength of plasmagel 

 and inhibited pseudopod formation, that there was no threshold for 

 response, and that the "all-or-none law" did not apply. The rate of 

 locomotion, however, was dependent upon both the intensity of illumi- 

 nation and the state of light adaptation of the ameba, with an opti- 

 mum reached at about 15,000 m-c after an exposure of some minutes. 

 Decrease in rate of movement with further increase in light intensity was 

 believed due to the "gelating action of the shorter waves of fight." 



Some doubt was thrown on the adequacy of these explanations by the 

 work of Folger (1927), in which the effect of sudden illumination in 

 inducing cessation of activity in amebas was found to be interchangeable 

 with the effect of mechanical shock; both showed a definite reaction time, 

 a latent period, and a refractory period. Mechanical shock affected the 

 reaction to light, except that, when an ameba formed a food cup, it failed 

 to respond to light stimulation, and after the cup was formed, the stimu- 

 lation period and reaction time were sometimes shorter than usual. After 

 a response to fight the ameba must recover before it will respond to shock. 

 Hence the two stimuli must act in the same way, and since temperature 

 has little, if any, effect on reaction time, the mechanism is quite obscure. 

 The work of Schaeffer (1914, 1917) makes understanding of the light 



