Photoreception 383 



one part of the animal is more intensely illuminated than another, the ani- 

 mal will move in the direction of the less intensely illuminated side. Under 

 uniform but sudden illumination the animal exhibits an increased locomotor 

 activity, which gradually subsides. The brief description accorded the latter 

 type of behavior gives the impression of random movement, which may 

 perhaps be called a kinesis, that is, an increase in random activity. 



Many annelids (e.g., Lumbricus^ possess no macroscopic photoreceptors, 

 yet in Lumhricus the entire body is sensitive to light. Sensitivity is greatest 

 on the prostomium, next greatest on the dorsal region of the next few an- 

 terior segments and a few of the most posterior segments, and least in the 

 middle of the body. The regions of greatest sensitivity coincide with the 

 highest concentration of the so-called epithelial "light cells" of Hess, which 

 are single cell photoreceptors each with a refractile body connected to neuro- 

 fibrillae. Hess^'^ found that the relative frequency of occurrence of these 

 receptors in certain segments is as follows: prostomium, 57; segment 1, 26; 

 segment 2, 10; segment 3, 7; segment 11, 1; last segment, 14; penultimate, 

 4; antepenultimate, 2. The distribution of light sensitivity thus parallels the 

 distribution of the light-sensitive cells. In some oligochaetes the photore- 

 ceptors are found directly on the nerves of the cerebral ganglia. Earth- 

 worms are positively phototaxic at very low intensities of illumination and 

 negatively phototaxic at higher intensities.^'^- ^^^ The type of orientation 

 which these worms exhibit most often is called klinotaxis (see below). 



The molluscs display a great diversity in the mechanism of photoreception, 

 ranging from a diffuse light sensitivity to a photoreceptor as complex as the 

 human eye. The diffuse sensitivity to light in the clam, Mya arenaria, is con- 

 fined to the siphon. Under constant conditions of illumination, the siphon 

 is extended; when illumination increases, the siphon is withdrawn. The 

 photoreceptors of the siphon are similar to those of the earthworm. ^^^* 



Localized Sensitivity to Light 



Localized photoreceptors are present in a great variety of organisms and ex- 

 hibit a great diversity of structure. For the sake of clarity it seems pertinent 

 to classify the photoreceptors on the basis of their gross function. Thus, cer- 

 tain eyes function only as intensity receptors, while others function in form 

 perception and pattern vision, the latter again forming a large group 

 of great morphological diversity. 



Morphology. The simplest type of light-sensitive organs appear among the 

 flagellate Protozoa. The known light-sensitive structures of the Protozoa are 

 stigmas and ocelli, which occur only in Hagellates. The stigma of Euglena 

 (Fig. 101) is a mass of red granules, which shade the swelling on the flagel- 

 lum. The flagellar swelling is the light-sensitive structure, and in any photo- 

 taxic organism change of illumination of this swelling causes a change in 

 direction of the beat of the flagellum until the organism becomes properly 

 oriented (Fig. 102). In the Phytomonadida the stigma consists of a refractile 

 structure which serves as a lens and covers the opening of a cup-shaped mass 

 of pigment. The light-sensitive material is between the lens and the 

 inner surface of the cup. The function of the pigment is to make the or- 

 ganelle a directional detector useful in phototaxis. The stigmas of fresh-water 

 Dinoflagellida are similar to those of Phytomonadida. However, those of cer- 



