624 RADIATION BIOLOGY 



did not alter with illumination, it was not involved in the fundamental 

 chemical equihbria. Eyes of the decapod crabs Carcinus and Uca failed 

 to yield any traces of retinene but contained high concentrations of vita- 

 min Ai. Bhss (1943, 1946) found that eye extracts from squid, the crab 

 Callinectes, and the xiphosuran Limulus were photostable until treated 

 with dilute formaldehyde but then bleached to release large quantities of 

 retinene. He introduced the term " cephalopsin " for the photostable 

 pigment of squid retinas and noted that, although its absorption spec- 

 trum was like that of rhodopsin in vertebrate eyes, it differed chemically. 

 St. George and Wald (1949) repeated this work with somewhat different 

 technique and obtained a bleachable pigment that showed no important 

 distinction from vertebrate rhodopsin. 



Other approaches to invertebrate photochemistry have been mostly 

 measures of spectral sensitivity in the intact animal. 



PHOTOSENSORY STRUCTURES 



From the anatomical side, numerous attempts have been made to keep 

 the record up to date (Carriere, 1885; Beer, 1901; Hesse, 1902; Parker, 

 1922; Plate, 1924; Uexkuell and Brock, 1927; Hess, 1943; Kahmann, 1947; 

 and the long series of short papers by Hilton, 1920-1941). Most of these 

 provide brief treatments of the gross histology of sense organs or receptor 

 cells and indicate the limitations inherent in each of the several levels of 

 organization. In general, however, only articles with a more limited 

 scope provide much information about the relation between structure 

 and function, presenting the subject in terms of the reacting animal. 



In the following account the terminology applied to each photosensory 

 organ is intended to indicate something of its structure. In protozoans 

 having a speciaUzed photoreceptor within the single cell, it is termed a 

 stigma. In multicellular animals the photoreceptor may be a specialized 

 neuron of the nervous system but may lack conspicuous pigments; this is 

 described as a neuronal photoreceptor. If the photosensory structure is 

 unicellular but has obvious pigment associated with it, giving its sensi- 

 tivity a measure of directionality, the word eyespot is applied. Multi- 

 cellular photoreceptors, by contrast, are called eyes. They may be ocelli, 

 or "simple eyes," if a number of receptor cells are grouped into a retina 

 consisting of more than a single circle ; light may reach the retina through 

 a pinhole in a pigment diaphragm (as in Nautilus), through the cavity of 

 an open, hollow cup (as in various gastropods), or through a single lens. 

 Or the eyes may be compound eyes, composed of individual om.matidia, 

 in which the receptor cells form a single circle like the segments of an 

 orange and light reaches them through a lens system along the axis of 

 symmetry of the group. 



Compound eyespots are known in some annelids and some mollusks. 



